Refactor compositor event handling path to be callback-based

content/renderer/input/input_handler_manager_client.h

Index: content/renderer/input/input_handler_manager_client.h
diff --git a/content/renderer/input/input_handler_manager_client.h b/content/renderer/input/input_handler_manager_client.h
index cdc38633eaeabe73c6c05d9519166f8e3db80dc3..49cf2b6aef054c01092aa1090e3234b0a9257c40 100644
--- a/content/renderer/input/input_handler_manager_client.h
+++ b/content/renderer/input/input_handler_manager_client.h
@@ -13,17 +13,13 @@
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 #include "ui/gfx/geometry/vector2d_f.h"
 
-namespace ui {
-class LatencyInfo;
-struct DidOverscrollParams;
-}
-
 namespace cc {
 class InputHandler;
 }
 
 namespace ui {
 class SynchronousInputHandlerProxy;
+struct DidOverscrollParams;
 }
 
 namespace content {
@@ -43,6 +39,10 @@ class CONTENT_EXPORT InputHandlerManagerClient {
   // Called from the compositor thread.
   virtual void RegisterRoutingID(int routing_id) = 0;
   virtual void UnregisterRoutingID(int routing_id) = 0;
+
+  // Overscroll triggered by |HandleInputEvent| will response to passed-in
+  // callback. (e.g. |InputEventFilter::DidForwardToHandlerAndOverscroll|)
+  // Otherwise |DidOverscroll| will be fired.
   virtual void DidOverscroll(int routing_id,
                              const ui::DidOverscrollParams& params) = 0;
   virtual void DidStartFlinging(int routing_id) = 0;