Implement a Hit Test Cache.

Source/platform/geometry/LayoutRect.h

Index: Source/platform/geometry/LayoutRect.h
diff --git a/Source/platform/geometry/LayoutRect.h b/Source/platform/geometry/LayoutRect.h
index bfe86fc69df82597a74d943458c7383b035a559e..ac4407bdd43e96c79077cb04e322e5225cd729dc 100644
--- a/Source/platform/geometry/LayoutRect.h
+++ b/Source/platform/geometry/LayoutRect.h
@@ -236,6 +236,10 @@ inline IntRect pixelSnappedIntRect(const LayoutRect& rect)
 PLATFORM_EXPORT IntRect enclosingIntRect(const LayoutRect&);
 PLATFORM_EXPORT LayoutRect enclosingLayoutRect(const FloatRect&);
 
+// Returns a layout rect with minimum size for width and height that contains
+// |point|.
+PLATFORM_EXPORT LayoutRect boundingRect(const LayoutPoint&);
+
 inline IntRect pixelSnappedIntRect(LayoutUnit left, LayoutUnit top, LayoutUnit width, LayoutUnit height)
 {
     return IntRect(left.round(), top.round(), snapSizeToPixel(width, left), snapSizeToPixel(height, top));