Blink Support for Overlay Scrollbar Animation

Source/platform/scroll/ScrollbarThemeClient.h

Index: Source/platform/scroll/ScrollbarThemeClient.h
diff --git a/Source/platform/scroll/ScrollbarThemeClient.h b/Source/platform/scroll/ScrollbarThemeClient.h
index fc8585fd0695866f4640f1264396cc9cfa6cf1cd..4079d800262c4408e117390a6bda9b92af863939 100644
--- a/Source/platform/scroll/ScrollbarThemeClient.h
+++ b/Source/platform/scroll/ScrollbarThemeClient.h
@@ -31,6 +31,7 @@
 #include "platform/geometry/IntRect.h"
 #include "platform/geometry/IntSize.h"
 #include "platform/scroll/ScrollTypes.h"
+#include "public/platform/WebThemeEngine.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/Vector.h"
 
@@ -84,6 +85,23 @@ public:
 
     virtual bool isOverlayScrollbar() const = 0;
 
+    virtual bool isDuringStateTransitionAnimation() const
+    {
+        return false;
+    }
+    virtual double stateTransitionProgress() const
+    {
+        return 0;
+    }
+    virtual blink::WebThemeEngine::State stateTransitionStartState() const
+    {
+        return blink::WebThemeEngine::StateNormal;
+    }
+    virtual blink::WebThemeEngine::State stateTransitionEndState() const
+    {
+        return blink::WebThemeEngine::StateNormal;
+    }
+
     virtual bool isAlphaLocked() const = 0;
     virtual void setIsAlphaLocked(bool) = 0;