Handle touchpad flings with passive event listeners on compositor thread.

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 ee1b2600962fd6794e505a2f1d204abbe4d6c02b..bd68cf951a0a43bf9c8dcccf622d66786b00b8e9 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -5,6 +5,8 @@
 #ifndef UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 #define UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 
+#include "ui/events/blink/scoped_web_input_event.h"
+
 namespace blink {
 class WebGestureCurve;
 struct WebActiveWheelFlingParameters;
@@ -25,6 +27,14 @@ class InputHandlerProxyClient {
   virtual void TransferActiveWheelFlingAnimation(
       const blink::WebActiveWheelFlingParameters& params) = 0;
 
+  // Gesture flings from touchpads are processed via a gesture curve
+  // animation on the compositor thread, and these events generate MouseWheel
+  // events which need to propagate to the main thread. If there are blocking
+  // event listeners the entire fling animation is moved to the main thread
+  // via |TransferActiveWheelFlingAnimation|.

[tdresser, 2016/09/14 18:58:08]

This comment doesn't obviously connect with the method its commenting on. Can
you make the connection clearer?

[dtapuska, 2016/09/15 20:43:04]

Done.

+  virtual void DispatchNonBlockingEventToMainThread(
+      ui::ScopedWebInputEvent event) = 0;
+
   // Creates a new fling animation curve instance for device |device_source|
   // with |velocity| and already scrolled |cumulative_scroll| pixels.
   virtual blink::WebGestureCurve* CreateFlingAnimationCurve(