Implement Wheel Gesture Scrolling on OSX.

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 9a3956264ec75c1179175524344b0c47ca66ed69..8bcb688303c03c1236b7f063a9dc7d45bc188dc4 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -99,7 +99,13 @@ class CC_EXPORT InputHandler {
     uint32_t main_thread_scrolling_reasons;
   };
 
-  enum ScrollInputType { GESTURE, WHEEL, ANIMATED_WHEEL, NON_BUBBLING_GESTURE };
+  enum ScrollInputType {
+    GESTURE,  // TODO(dtapuska): Remove this and just use TOUCHSCREEN.

[tdresser, 2016/03/08 14:28:52]

I'm a fan of giving TODOs associated bugs and referencing them in the comments -
it makes it easier to tell that you've done all the associated cleanup.

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

Done.

+    TOUCHSCREEN = GESTURE,
+    WHEEL,
+    ANIMATED_WHEEL,
+    NON_BUBBLING_GESTURE
+  };
 
   // Binds a client to this handler to receive notifications. Only one client
   // can be bound to an InputHandler. The client must live at least until the
@@ -118,6 +124,12 @@ class CC_EXPORT InputHandler {
   // targets at the root layer.
   virtual ScrollStatus RootScrollBegin(ScrollState* scroll_state,
                                        ScrollInputType type) = 0;
+
+  // Returns SCROLL_ON_IMPL_THREAD if a layer is actively being scrolled or
+  // a subsequent call to ScrollAnimated can begin on the impl thread.
+  virtual ScrollStatus ScrollAnimatedBegin(
+      const gfx::Point& viewport_point) = 0;
+
   virtual ScrollStatus ScrollAnimated(const gfx::Point& viewport_point,
                                       const gfx::Vector2dF& scroll_delta) = 0;
 
@@ -137,7 +149,7 @@ class CC_EXPORT InputHandler {
   virtual bool ScrollVerticallyByPage(const gfx::Point& viewport_point,
                                       ScrollDirection direction) = 0;
 
-  // Returns SCROLL_STARTED if a layer was being actively being scrolled,
+  // Returns SCROLL_STARTED if a layer was actively being scrolled,
   // SCROLL_IGNORED if not.
   virtual ScrollStatus FlingScrollBegin() = 0;