Refactor compositor event handling path to be callback-based

ui/events/blink/input_handler_proxy_client.h

Index: ui/events/blink/input_handler_proxy_client.h
diff --git a/ui/events/blink/input_handler_proxy_client.h b/ui/events/blink/input_handler_proxy_client.h
index efc24ed6d17d4a83b8879422c96db83481f7d89a..ee1b2600962fd6794e505a2f1d204abbe4d6c02b 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -32,6 +32,9 @@ class InputHandlerProxyClient {
       const blink::WebFloatPoint& velocity,
       const blink::WebSize& cumulative_scroll) = 0;
 
+  // |HandleInputEvent/WithLatencyInfo| will respond to overscroll by calling
+  // the passed in callback.
+  // Otherwise |DidOverscroll| will be fired.
   virtual void DidOverscroll(
       const gfx::Vector2dF& accumulated_overscroll,
       const gfx::Vector2dF& latest_overscroll_delta,