Implement a Hit Test Cache.

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 941 matching lines @@
     if (protector->hasOneRef())
         return;
 #endif
 
     Document* document = m_frame->document();
 
     // If the layout view was marked as needing layout after we added items in the subtree roots we need
     // to clear the roots and do the layout from the layoutView.
     if (layoutView()->needsLayout())
         clearLayoutSubtreeRootsAndMarkContainingBlocks();
+    layoutView()->clearHitTestCache();

[esprehn, 2015/05/25 21:01:45]

This is not enough, you also need to invalidate on style, dom modification, and
scroll...

 
     bool inSubtreeLayout = isSubtreeLayout();
 
     // FIXME: The notion of a single root for layout is no longer applicable. Remove or update this code. crbug.com/460596
     LayoutObject* rootForThisLayout = inSubtreeLayout ? *(m_layoutSubtreeRoots.begin()) : layoutView();
     if (!rootForThisLayout) {
         // FIXME: Do we need to set m_size here?
         ASSERT_NOT_REACHED();
         return;
     }
@@ skipping 3096 matching lines @@
 
     if (!graphicsLayer)
         return;
 
     DeprecatedPaintLayer::mapRectToPaintInvalidationBacking(localFrame->contentLayoutObject(), paintInvalidationContainer, viewRect);
 
     graphicsLayerTimingRequests.add(graphicsLayer, Vector<std::pair<int64_t, WebRect>>()).storedValue->value.append(std::make_pair(m_frame->frameID(), enclosingIntRect(viewRect)));
 }
 
 } // namespace blink