Implement a Hit Test Cache.

Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1781 matching lines @@
 
     if (diff.transformChanged() && !needsLayout()) {
         if (LayoutBlock* container = containingBlock())
             container->setNeedsOverflowRecalcAfterStyleChange();
     }
 
     if (updatedDiff.needsPaintInvalidationLayer())
         toLayoutBoxModelObject(this)->layer()->setShouldDoFullPaintInvalidationIncludingNonCompositingDescendants();
     else if (diff.needsPaintInvalidationObject() || updatedDiff.needsPaintInvalidationObject())
         setShouldDoFullPaintInvalidation();
+
+    view()->clearHitTestCache();

[Rick Byers, 2015/06/05 20:48:59]

I don't know enough about the style system to say for sure that this is the
right place.  Hopefully esprehn will chime in, but if not and manual testing
isn't showing any false cache hits, then go ahead and land it and we'll see what
we get from UMA :-)

[esprehn, 2015/06/06 21:14:30]

You don't want to be here, this puts code inside setStyle() for every element on
the page, you want to be in Document::updateLayoutTree.

[dtapuska, 2015/06/09 18:21:24]

Done.

[dtapuska, 2015/06/09 18:21:24]

Done.

 }
 
 static inline bool layoutObjectHasBackground(const LayoutObject* layoutObject)
 {
     return layoutObject && layoutObject->hasBackground();
 }
 
 void LayoutObject::styleWillChange(StyleDifference diff, const ComputedStyle& newStyle)
 {
     if (m_style) {
@@ skipping 1488 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif