Expand tap highlight to allow multiple highlights for touch disambiguation.

Source/core/platform/graphics/GraphicsLayer.h

Index: Source/core/platform/graphics/GraphicsLayer.h
diff --git a/Source/core/platform/graphics/GraphicsLayer.h b/Source/core/platform/graphics/GraphicsLayer.h
index ae7a045774ea5a366ad41b24baa51a5e57a9540e..214e2398fb1fe5218457a2f3243c01333f204dfd 100644
--- a/Source/core/platform/graphics/GraphicsLayer.h
+++ b/Source/core/platform/graphics/GraphicsLayer.h
@@ -41,6 +41,7 @@
 #include "wtf/HashMap.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
+#include "wtf/Vector.h"
 
 #include "public/platform/WebAnimationDelegate.h"
 #include "public/platform/WebCompositingReasons.h"
@@ -408,9 +409,10 @@ public:
         return false;
     }
 
-    void setLinkHighlight(LinkHighlightClient*);
+    void addLinkHighlight(LinkHighlightClient*);
+    void removeLinkHighlight(LinkHighlightClient*);
     // Exposed for tests
-    LinkHighlightClient* linkHighlight() { return m_linkHighlight; }
+    LinkHighlightClient* linkHighlight(int i) { return m_linkHighlights[i]; }
 
     void setScrollableArea(ScrollableArea*, bool isMainFrame);
     ScrollableArea* scrollableArea() const { return m_scrollableArea; }
@@ -533,7 +535,7 @@ protected:
     // on.
     int m_contentsLayerId;
 
-    LinkHighlightClient* m_linkHighlight;
+    Vector<LinkHighlightClient*> m_linkHighlights;
 
     OwnPtr<OpaqueRectTrackingContentLayerDelegate> m_opaqueRectTrackingContentLayerDelegate;