Turn off selection gap painting, but allow toggling via a runtime flag

LayoutTests/TestExpectations

 # turn off Slimming Paint tests for all non-Linux platforms...
 crbug.com/453972 [ Mac ] virtual/slimmingpaint/ [ Skip ]
 crbug.com/453972 [ Win ] virtual/slimmingpaint/ [ Skip ]
 crbug.com/453972 [ Android ] virtual/slimmingpaint/ [ Skip ]
 # except for fast/block/, as a sanity test directory.
 crbug.com/453972 [ Mac ] virtual/slimmingpaint/fast/block [ Pass ]
 crbug.com/453972 [ Win ] virtual/slimmingpaint/fast/block [ Pass ]
 crbug.com/453972 [ Android ] virtual/slimmingpaint/fast/block [ Pass ]
 
 crbug.com/452320 [ Mac ] virtual/slimmingpaint/fast/scrolling/scrollbar-tickmarks-hittest.html [ Timeout ]
@@ skipping 191 matching lines @@
 crbug.com/449242 virtual/slimmingpaint/fast/css/word-spacing-characters-complex-text.html [ Pass ImageOnlyFailure Crash ]
 
 crbug.com/438589 fast/repaint/obscured-background-no-repaint.html [ Failure Pass ]
 crbug.com/438589 virtual/slimmingpaint/fast/repaint/obscured-background-no-repaint.html [ Failure Pass ]
 
 # Crashing after V8 roll to 3.28.43.1.
 crbug.com/398306 fast/events/before-unload-return-bad-value.html [ Crash Timeout Pass ]
 
 crbug.com/234069 [ Mac Release ] fast/events/touch/gesture/long-press-drag-drop-touch-editing-combined.html [ Failure Pass ]
 
+crbug.com/471908 editing/deleting/delete-to-select-table.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/4916541.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/5136770.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/5142012-1.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/5142012-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/5190926.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/5481523.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/indent-selection.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/remove-list-from-range-selection.html [ NeedsRebaseline ]
+crbug.com/471908 editing/execCommand/selectAll.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/3690703-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/3690703.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/3690719.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/4818145.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/4947387.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/5057506-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/5057506.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/5232159.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/7152-1.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/7152-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/doubleclick-crash.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/extend-by-character-002.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/extend-by-character-003.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/image-before-linebreak.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/linux_selection_color.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/mixed-editability-6.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/mixed-editability-7.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/node-removal-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/paragraph-granularity.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/range-between-block-and-inline.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/replaced-boundaries-3.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-001.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-002.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-003.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-004.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-005.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-all-006.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/select-missing-image.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/selection-actions.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/selection-button-text.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-lr-ltr-extend-line-backward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-lr-ltr-extend-line-forward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-backward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-backward-p.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-backward-wrap.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-forward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-forward-p.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-ltr-extend-line-forward-wrap.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-rtl-extend-line-backward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-rtl-extend-line-backward-p.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-rtl-extend-line-forward-br.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/vertical-rl-rtl-extend-line-forward-p.html [ NeedsRebaseline ]
+crbug.com/471908 editing/selection/word-granularity.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/5017613-1.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/5017613-2.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/5046875-1.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/5228141.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/5279521.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/apple-style-editable-mix.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/block-style-003.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/create-block-for-style-005.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/create-block-for-style-006.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/create-block-for-style-010.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/create-block-for-style-011.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/create-block-for-style-012.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/relative-font-size-change-001.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/relative-font-size-change-002.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/style-3998892-fix.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/style-boundary-004.html [ NeedsRebaseline ]
+crbug.com/471908 editing/style/table-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/block/line-layout/selection-highlight-overlap.html [ NeedsRebaseline ]
+crbug.com/471908 fast/lists/markers-in-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/overflow/image-selection-highlight.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/block-selection-gap-in-composited-layer.html [ NeedsRebaseline ]
+crbug.com/471908 [ Linux Mac ] fast/ruby/select-ruby.html [ NeedsRebaseline ]
+crbug.com/471908 fast/sub-pixel/selection/selection-gaps-at-fractional-offsets.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/selection-hard-linebreak.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/selection-multiple-runs.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/selection-rect-rounding.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/shaping/shaping-selection-rect.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/whitespace/pre-wrap-overflow-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/whitespace/select-new-line-with-line-break-normal.html [ NeedsRebaseline ]
+crbug.com/471908 compositing/overflow/selection-gaps-after-removing-scrolling-contents.html [ NeedsRebaseline ]
+crbug.com/471908 compositing/overflow/selection-gaps-toggling-with-scrolling-contents.html [ NeedsRebaseline ]
+crbug.com/471908 compositing/overflow/selection-gaps-toggling.html [ NeedsRebaseline ]
+crbug.com/471908 compositing/squashing/selection-repaint-with-gaps.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/japanese-rl-selection-clear.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/japanese-rl-selection-repaint.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-after-delete.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-change-in-iframe-with-relative-parent.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-clear.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-flipped-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-flipped-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-transformed-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-gap-transformed-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 fast/repaint/selection-partial-invalidation-between-blocks.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/prefer_compositing_to_lcd_text/compositing/overflow/selection-gaps-after-removing-scrolling-contents.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/prefer_compositing_to_lcd_text/compositing/overflow/selection-gaps-toggling-with-scrolling-contents.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/prefer_compositing_to_lcd_text/compositing/overflow/selection-gaps-toggling.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/japanese-rl-selection-clear.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/japanese-rl-selection-repaint.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-after-delete.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-change-in-iframe-with-relative-parent.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-clear.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-flipped-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-flipped-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-transformed-absolute-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-gap-transformed-fixed-child.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/selection-partial-invalidation-between-blocks.html [ NeedsRebaseline ]
+crbug.com/471908 fast/forms/form-added-to-table.html [ NeedsRebaseline ]
+crbug.com/471908 fast/text/emphasis.html [ NeedsRebaseline ]
+crbug.com/471908 fast/writing-mode/horizontal-bt-replaced-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/writing-mode/japanese-lr-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/writing-mode/japanese-rl-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/writing-mode/vertical-lr-replaced-selection.html [ NeedsRebaseline ]
+crbug.com/471908 fast/writing-mode/vertical-rl-replaced-selection.html [ NeedsRebaseline ]
+crbug.com/471908 svg/text/foreignObject-text-clipping-bug.xml [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/overflow/image-selection-highlight.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/repaint/block-selection-gap-in-composited-layer.html  [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/text/selection-hard-linebreak.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/text/selection-rect-rounding.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/text/shaping/shaping-selection-rect.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/text/whitespace/pre-wrap-overflow-selection.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/fast/text/whitespace/select-new-line-with-line-break-normal.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/slimmingpaint/svg/text/foreignObject-text-clipping-bug.xml [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/emphasis.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/selection-hard-linebreak.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/selection-painted-separately.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/selection-rect-rounding.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/shaping/shaping-selection-rect.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/whitespace/pre-wrap-overflow-selection.html [ NeedsRebaseline ]
+crbug.com/471908 virtual/antialiasedtext/fast/text/whitespace/select-new-line-with-line-break-normal.html [ NeedsRebaseline ] 
+crbug.com/471908 virtual/slimmingpaint/fast/block/line-layout/selection-highlight-overlap.html [ NeedsRebaseline ]
+
 crbug.com/330083 imported/web-platform-tests/shadow-dom/elements-and-dom-objects/extensions-to-element-interface/methods/elements-001.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/elements-and-dom-objects/extensions-to-element-interface/methods/non-element-nodes-001.html [ Failure ]
 crbug.com/442632 imported/web-platform-tests/shadow-dom/elements-and-dom-objects/extensions-to-event-interface/event-path-001.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/elements-and-dom-objects/shadowroot-object/shadowroot-methods/test-004.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/events/event-dispatch/test-001.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/events/event-dispatch/test-003.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/events/event-retargeting/test-004.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/events/retargeting-relatedtarget/test-003.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/html-elements-and-their-shadow-trees/test-001.html [ Failure ]
 crbug.com/330083 imported/web-platform-tests/shadow-dom/html-elements-and-their-shadow-trees/test-002.html [ Failure ]
@@ skipping 1601 matching lines @@
 crbug.com/418091 [ SnowLeopard Debug ] css2.1/t100801-c544-valgn-03-d-agi.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard Debug ] css2.1/t100801-c544-valgn-04-d-agi.html [ Pass Failure ]
 crbug.com/418091 [ Mac ] fast/text/aat-morx.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard Debug ] virtual/antialiasedtext/fast/text/aat-morx.html [ Failure ]
 crbug.com/418091 [ SnowLeopard Debug ] virtual/antialiasedtext/fast/text/cg-fallback-bolding.html [ Failure ]
 crbug.com/418091 [ SnowLeopard Release ] virtual/antialiasedtext/fast/text/decorations-transformed.html [ Failure ]
 crbug.com/418091 [ SnowLeopard Release ] virtual/antialiasedtext/fast/text/emphasis-avoid-ruby.html [ Failure ]
 crbug.com/418091 [ SnowLeopard Release ] virtual/antialiasedtext/fast/text/fixed-pitch-control-characters.html [ Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/justify-nbsp.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/justify-padding-distribution.html [ Pass Failure ]
-crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/selection-painted-separately.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/setData-dirty-lines.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/soft-hyphen-2.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/soft-hyphen-3.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/soft-hyphen-4.html [ Pass Failure ]
 crbug.com/418091 [ SnowLeopard ] virtual/antialiasedtext/fast/text/splitText-dirty-lines.html [ Pass Failure ]
 
 crbug.com/446385 [ Linux Mac ] http/tests/xmlhttprequest/xmlhttprequest-json-response-overflow.html [ Pass Timeout ]
 crbug.com/446385 [ Win7 Debug ] http/tests/xmlhttprequest/xmlhttprequest-json-response-overflow.html [ Crash Pass Timeout ]
 
 crbug.com/442512 svg/dom/viewspec-aspectparams.html [ Pass ImageOnlyFailure ]
@@ skipping 269 matching lines @@
 crbug.com/473632 fast/dom/Geolocation/success-clear-watch.html [ Pass Timeout ]
 
 crbug.com/472905 inspector/elements/styles/page-reload-update-sidebar.html [ Pass Failure ]
 
 crbug.com/473718 http/tests/navigation/beacon-cross-origin-redirect.html [ Pass Failure ]
 crbug.com/473722 [ XP ] http/tests/fetch/workers/stream-reader.html [ Slow ]
 crbug.com/473725 [ Linux ] virtual/slimmingpaint/fast/forms/submit-change-fragment.html [ Pass Timeout ]
 
 crbug.com/474224 [ Mac ] inspector-protocol/input/dispatchKeyEvent-focus.html [ Failure ]
 crbug.com/453822 virtual/slimmingpaint/fast/forms/calendar-picker/datepicker-crash-after-keydown.html [ Crash Pass Timeout ]