DevTools: fix inspector/console/console-viewport-selection.html flakiness

LayoutTests/inspector/console/console-viewport-selection-expected.txt

 CONSOLE MESSAGE: line 9: Message #0
 CONSOLE MESSAGE: line 9: Message #1
 CONSOLE MESSAGE: line 9: Message #2
 CONSOLE MESSAGE: line 9: Message #3
 CONSOLE MESSAGE: line 9: Message #4
 CONSOLE MESSAGE: line 9: Message #5
 CONSOLE MESSAGE: line 9: Message #6
 CONSOLE MESSAGE: line 9: Message #7
 CONSOLE MESSAGE: line 9: Message #8
 CONSOLE MESSAGE: line 9: Message #9
@@ skipping 140 matching lines @@
 Tests that console viewport handles selection properly.
 
 
 Running: verifyViewportIsTallEnough
 Viewport contains 13 messages
 
 Running: testSelectionSingleLineText
 Selected text:<<<EOL
 ssage
 EOL
-first visible message index: 63   viewport is covered: YES
+first visible message index: 75   viewport is covered: YES
 
 Running: testReversedSelectionSingleLineText
 Selected text:<<<EOL
 ssage
 EOL
-first visible message index: 63   viewport is covered: YES
+first visible message index: 75   viewport is covered: YES
 
 Running: testSelectionMultiLineText
 Selected text:<<<EOL
 age #73 console-viewport-selection.html:9
 Message #74 console-viewport-selection.html:9
 Message
 EOL
-first visible message index: 64   viewport is covered: YES
+first visible message index: 74   viewport is covered: YES
 
 Running: testSimpleVisibleSelection
 anchor = {item: 72, offset: 6}, head = {item: 77, offset: 6}
-first visible message index: 65   viewport is covered: YES
+first visible message index: 72   viewport is covered: YES
 
 Running: testHalfScrollSelectionUp
 anchor = {item: 72, offset: 6}, head = {item: 77, offset: 6}
 first visible message index: 75   viewport is covered: YES
 
 Running: testHalfScrollSelectionDown
 anchor = {item: 72, offset: 6}, head = {item: 77, offset: 6}
 first visible message index: 63   viewport is covered: YES
 
 Running: testScrollSelectionAwayUp
@@ skipping 11 matching lines @@
 Running: testShiftClickSelectionBelow
 anchor = {item: 72, offset: 6}, head = {item: 140, offset: 0}
 first visible message index: 135   viewport is covered: YES
 
 Running: testRemoveSelection
 anchor = null, head = null
 first visible message index: 135   viewport is covered: YES
 
 Running: testReversedVisibleSelection
 anchor = {item: 76, offset: 6}, head = {item: 71, offset: 6}
-first visible message index: 64   viewport is covered: YES
+first visible message index: 71   viewport is covered: YES
 
 Running: testShiftClickReversedSelectionOver
 anchor = {item: 76, offset: 6}, head = {item: 10, offset: 0}
 first visible message index: 5   viewport is covered: YES
 
 Running: testShiftClickReversedSelectionBelow
 anchor = {item: 76, offset: 6}, head = {item: 140, offset: 0}
 first visible message index: 135   viewport is covered: YES