Implement a Hit Test Cache.

Source/core/layout/LayoutObject.cpp

Index: Source/core/layout/LayoutObject.cpp
diff --git a/Source/core/layout/LayoutObject.cpp b/Source/core/layout/LayoutObject.cpp
index f668620b095015648d489dd76352b72f812f59c8..0e209909ba71cc0a397c6ec76c0bb406c1413139 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -1799,6 +1799,8 @@ void LayoutObject::setStyle(PassRefPtr<ComputedStyle> style)
         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)