Allow proper highlighting on universal overflow scroll.

Source/core/rendering/RenderLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 381 matching lines @@
     frame->eventHandler().dispatchFakeMouseMoveEventSoonInQuad(quadForFakeMouseMoveEvent);
 
     bool requiresRepaint = true;
 
     if (box().view()->compositor()->inCompositingMode()) {
         // Hits in virtual/gpu/fast/canvas/canvas-scroll-path-into-view.html.
         DisableCompositingQueryAsserts disabler;
         bool onlyScrolledCompositedLayers = scrollsOverflow()
             && !layer()->hasVisibleNonLayerContent()
             && !layer()->hasNonCompositedChild()
-            && !layer()->hasBlockSelectionGapBounds()

[hartmanng, 2014/06/13 14:51:38]

This is no longer needed, because the block selection gaps bounds will now never
have to repaint on scroll. They will either be part of the scrolling contents
layer, or part of the block selection layer, but never part of the background.

This removal should also fix the scroll-update chicken-egg problems, so this
patch shouldn't be blocked anymore.

[hartmanng, 2014/06/13 16:43:56]

After talking with Ian offline, I think I was wrong about this. I misunderstood
what was going on here. This bool is only relevant if we're _not_ using
composited scrolling, so my patch shouldn't affect the behaviour here.

             && !box().isMarquee();
 
         if (usesCompositedScrolling() || onlyScrolledCompositedLayers)
             requiresRepaint = false;
     }
 
     // Just schedule a full repaint of our object.
     if (requiresRepaint) {
         if (RuntimeEnabledFeatures::repaintAfterLayoutEnabled()) {
             if (box().frameView()->isInPerformLayout())
@@ skipping 1066 matching lines @@
     DisableCompositingQueryAsserts disabler;
     return box().hasCompositedLayerMapping() && box().compositedLayerMapping()->scrollingLayer();
 }
 
 bool RenderLayerScrollableArea::needsCompositedScrolling() const
 {
     return scrollsOverflow() && box().view()->compositor()->acceleratedCompositingForOverflowScrollEnabled();
 }
 
 } // Namespace WebCore