Avoid sub-tree style recalc for ::selection.

LayoutTests/fast/css/invalidation/selection-pseudo-expected.txt

Index: LayoutTests/fast/css/invalidation/selection-pseudo-expected.txt
diff --git a/LayoutTests/fast/css/invalidation/selection-pseudo-expected.txt b/LayoutTests/fast/css/invalidation/selection-pseudo-expected.txt
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+CONSOLE ERROR: Discontiguous selection is not supported.
+Style invalidation for ::selection
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS getComputedStyle(select[0], '::selection').backgroundColor is red
+PASS internals.updateStyleAndReturnAffectedElementCount() is 2
+PASS getComputedStyle(select[0], '::selection').backgroundColor is green
+PASS [object Object] is defined.
+PASS getComputedStyle(select[1], '::selection').backgroundColor is red
+PASS internals.updateStyleAndReturnAffectedElementCount() is 2
+PASS getComputedStyle(select[1], '::selection').backgroundColor is green
+PASS successfullyParsed is true
+
+TEST COMPLETE
+1
+2
+