Implement a Hit Test Cache.

Source/core/layout/LayoutView.h

Index: Source/core/layout/LayoutView.h
diff --git a/Source/core/layout/LayoutView.h b/Source/core/layout/LayoutView.h
index 45d9a416b822c5fba03b600a3f430fe290984a52..b6c6e0108cb67f81139b6b6384ac99b15ed1cc75 100644
--- a/Source/core/layout/LayoutView.h
+++ b/Source/core/layout/LayoutView.h
@@ -25,6 +25,8 @@
 #include "core/CoreExport.h"
 #include "core/dom/Position.h"
 #include "core/frame/FrameView.h"
+#include "core/layout/HitTestCache.h"
+#include "core/layout/HitTestResult.h"
 #include "core/layout/LayoutBlockFlow.h"
 #include "core/layout/LayoutState.h"
 #include "core/layout/PaintInvalidationState.h"
@@ -51,10 +53,12 @@ public:
     void willBeDestroyed() override;
 
     bool hitTest(HitTestResult&);
-    bool hitTest(const HitTestRequest&, const HitTestLocation&, HitTestResult&);
 
     // Returns the total count of calls to HitTest, for testing.
     unsigned hitTestCount() const { return m_hitTestCount; }
+    unsigned hitTestCacheHits() const { return m_hitTestCacheHits; }
+
+    void clearHitTestCache() { m_hitTestCache.clear(); }
 
     virtual const char* name() const override { return "LayoutView"; }
 
@@ -211,6 +215,8 @@ private:
     unsigned m_layoutCounterCount;
 
     unsigned m_hitTestCount;
+    unsigned m_hitTestCacheHits;
+    HitTestCache m_hitTestCache;

[sof, 2015/06/09 18:59:18]

Oilpan-wise, to make this work on LayoutView, it needs to be handled&represented
like PendingSelection below. i.e., HitTestCache needs to be on the heap. (If
LayoutView had been on the heap itself, keeping it as a part object would have
been fine.)

[dtapuska, 2015/06/09 19:39:15]

In the posted version:
The pointers inside the HitTestResult are in fact raw ptrs which are really weak
because and are checked based on the dom tree version; if it doesn't match they
aren't used. So stale pointers won't get used.

Does turning HitTestCache into a GarbageCollected object imply that I need to
turn HitTestResult into a GarbageCollected object as well; that way I could
trace them and have the WeakMembers inside that class? 

If HitTestResult is created on the stack in a few places.. does it imply that I
need to turn all those stack allocations into heap allocations too?

 
     OwnPtrWillBePersistent<PendingSelection> m_pendingSelection;
 };