MacViews: Send Mac scrollWheel NSEvents as ui::ET_SCROLL (ui::ScrollEvent).

ui/events/event_constants.h

Index: ui/events/event_constants.h
diff --git a/ui/events/event_constants.h b/ui/events/event_constants.h
index 840631bc250719da17c7ca53b8bfa995c8f2f4de..c7736622783d2335da227a73f470beeaa3012a38 100644
--- a/ui/events/event_constants.h
+++ b/ui/events/event_constants.h
@@ -164,6 +164,25 @@ enum EventPhase {
   EP_POSTDISPATCH
 };
 
+// Momentum phase information used for a ScrollEvent.
+enum class EventMomentumPhase {
+  // Event is a non-momentum update to an event stream already begun.
+  NONE,
+
+  // Event is the beginning of an event stream that may result in momentum.
+  MAY_BEGIN,
+
+  // Event is an update while in a momentum phase. A "begin" event for the
+  // momentum phase portion of an event stream uses this also, but the scroll
+  // offsets will be zero.
+  INERTIAL_UPDATE,
+
+  // Event marks the end of the current event stream. Note that this is also set
+  // for events that are not a "stream", but indicate both the start and end of
+  // the event (e.g. a mouse wheel tick).
+  END,
+};
+
 // Device ID for Touch and Key Events.
 enum EventDeviceId {
   ED_UNKNOWN_DEVICE = -1