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

Source/core/layout/LayoutBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 2825 matching lines @@
 IntRect alignSelectionRectToDevicePixels(LayoutRect& rect)
 {
     LayoutUnit roundedX = rect.x().round();
     return IntRect(roundedX, rect.y().round(),
         (rect.maxX() - roundedX).round(),
         snapSizeToPixel(rect.height(), rect.y()));
 }
 
 bool LayoutBlockFlow::shouldPaintSelectionGaps() const
 {
-    return selectionState() != SelectionNone && style()->visibility() == VISIBLE && isSelectionRoot();
+    // TODO(chrishtr): remove the feature once we are sure it will be ok to do so. See crbug.com/471908 for more details.
+    return RuntimeEnabledFeatures::selectionGapPaintingEnabled()
+        && selectionState() != SelectionNone && style()->visibility() == VISIBLE && isSelectionRoot();
 }
 
 LayoutRect LayoutBlockFlow::blockSelectionGap(const LayoutBlock* rootBlock, const LayoutPoint& rootBlockPhysicalPosition, const LayoutSize& offsetFromRootBlock,
     LayoutUnit lastLogicalTop, LayoutUnit lastLogicalLeft, LayoutUnit lastLogicalRight, LayoutUnit logicalBottom, const PaintInfo* paintInfo) const
 {
     LayoutUnit logicalTop = lastLogicalTop;
     LayoutUnit logicalHeight = rootBlock->blockDirectionOffset(offsetFromRootBlock) + logicalBottom - logicalTop;
     if (logicalHeight <= 0)
         return LayoutRect();
 
@@ skipping 304 matching lines @@
     }
     if (isAnonymous())
         return "LayoutBlockFlow (anonymous)";
     if (isRelPositioned())
         return "LayoutBlockFlow (relative positioned)";
     return "LayoutBlockFlow";
 }
 
 
 } // namespace blink