Implement a Hit Test Cache.

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 2648 matching lines @@
     const FloatingObjectSet& floatingObjectSet = m_floatingObjects->set();
     FloatingObjectSetIterator begin = floatingObjectSet.begin();
     for (FloatingObjectSetIterator it = floatingObjectSet.end(); it != begin;) {
         --it;
         FloatingObject* floatingObject = it->get();
         if (floatingObject->shouldPaint() && !floatingObject->layoutObject()->hasSelfPaintingLayer()) {
             LayoutUnit xOffset = xPositionForFloatIncludingMargin(floatingObject) - floatingObject->layoutObject()->location().x();
             LayoutUnit yOffset = yPositionForFloatIncludingMargin(floatingObject) - floatingObject->layoutObject()->location().y();
             LayoutPoint childPoint = flipFloatForWritingModeForChild(floatingObject, adjustedLocation + LayoutSize(xOffset, yOffset));
             if (floatingObject->layoutObject()->hitTest(result, locationInContainer, childPoint)) {
-                updateHitTestResult(result, locationInContainer.point() - toLayoutSize(childPoint));
+                // TODO(dtapuska): calculate the layout rect
+                updateHitTestResult(result, locationInContainer.point() - toLayoutSize(childPoint), result.validityRect());
                 return true;
             }
         }
     }
 
     return false;
 }
 
 LayoutUnit LayoutBlockFlow::logicalLeftFloatOffsetForLine(LayoutUnit logicalTop, LayoutUnit fixedOffset, LayoutUnit logicalHeight) const
 {
@@ skipping 470 matching lines @@
     FrameView* frameView = document().view();
     LayoutUnit top = (style()->position() == FixedPosition) ? 0 : frameView->scrollOffset().height();
     int visibleHeight = frameView->visibleContentRect(IncludeScrollbars).height();
     if (size().height() < visibleHeight)
         top += (visibleHeight - size().height()) / 2;
     setY(top);
     dialog->setCentered(top);
 }
 
 } // namespace blink