Implement Wheel Gesture Scrolling on OSX.

ui/events/blink/input_scroll_elasticity_controller.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_EVENTS_BLINK_INPUT_SCROLL_ELASTICITY_CONTROLLER_H_
 #define UI_EVENTS_BLINK_INPUT_SCROLL_ELASTICITY_CONTROLLER_H_
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "cc/input/scroll_elasticity_helper.h"
@@ skipping 40 matching lines @@
   base::WeakPtr<InputScrollElasticityController> GetWeakPtr();
 
   // Update the overscroll state based a wheel event that has been processed.
   // Note that this assumes that all events are coming from a single input
   // device. If the user simultaneously uses multiple input devices, Cocoa may
   // not correctly pass all the gesture begin and end events. In this case,
   // this class may disregard some scrolls that come in at unexpected times.
   void ObserveWheelEventAndResult(
       const blink::WebMouseWheelEvent& wheel_event,
       const cc::InputHandlerScrollResult& scroll_result);
+  // Update the overscroll state based a gesture event that has been processed.
+  // Note that this assumes that all events are coming from a single input
+  // device. If the user simultaneously uses multiple input devices, Cocoa may
+  // not correctly pass all the gesture begin and end events. In this case,
+  // this class may disregard some scrolls that come in at unexpected times.

[tdresser, 2016/03/08 14:28:53]

How hard would it be to have a test of this behavior, ensuring that things don't
explode? I guess we'd have to characterize the unreliable Cocoa behavior, which
probably isn't worth it?

[dtapuska, 2016/03/08 20:31:49]

I think the behavior is that if you are using two input devices the phase
information will be interleaved and could be interpreted at incorrect times. The
mouse wheel event queue will try to make something sane out of it. But it could
certainly confused this class; this comment was copied from the
ObserveWheelEventAndResult because I feel it still applies. 

+  void ObserveGestureEventAndResult(
+      const blink::WebGestureEvent& gesture_event,
+      const cc::InputHandlerScrollResult& scroll_result);
+
   void Animate(base::TimeTicks time);
 
   void ReconcileStretchAndScroll();
 
  private:
   enum State {
     // The initial state, during which the overscroll amount is zero and
     // there are no active or momentum scroll events coming in. This state
     // is briefly returned to between the active and momentum phases of a
     // scroll (if there is no overscroll).
@@ skipping 67 matching lines @@
   // behavior as would happen if the scroll were caused by an active scroll).
   bool momentum_animation_reset_at_next_frame_;
 
   base::WeakPtrFactory<InputScrollElasticityController> weak_factory_;
   DISALLOW_COPY_AND_ASSIGN(InputScrollElasticityController);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_BLINK_INPUT_SCROLL_ELASTICITY_CONTROLLER_H_