Implement Wheel Gesture Scrolling on OSX.

third_party/WebKit/public/web/WebInputEvent.h

Index: third_party/WebKit/public/web/WebInputEvent.h
diff --git a/third_party/WebKit/public/web/WebInputEvent.h b/third_party/WebKit/public/web/WebInputEvent.h
index 3be5b18ebb24d3d90a822ec613be11e02767e113..ad6d1c6c4b639ae5b395e40bfdea57da66dadcf6 100644
--- a/third_party/WebKit/public/web/WebInputEvent.h
+++ b/third_party/WebKit/public/web/WebInputEvent.h
@@ -513,6 +513,16 @@ public:
             // If true, this event will skip hit testing to find a scroll
             // target and instead just scroll the viewport.
             bool targetViewport;
+            // If true, this event comes after a non-inertial gesture
+            // scroll sequence; OSX has unique phases for normal and
+            // momentum scroll events. Should always be false for touch based
+            // input as it generates GestureFlingStart instead.
+            bool inertial;
+            // True if the begin was generated when phase information is available
+            // for touchpad and for every scroll update we are generating a GSB/GSU/GSE
+            // tuple. This is a temporary variable that forces the non-latching behavior
+            // until crbug.com/526463 is fully implemented.

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

Mentioning that these synthesized begin events are used to force a hit test
might make this a bit clearer. 

Incorporating something like the following perhaps:

True if this event was synthesized in order to force a hit test, to avoid scroll
latching behavior until crbug.com/526463 is fully implemented.

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

Done.

+            bool synthetic;
         } scrollBegin;
 
         struct {
@@ -536,6 +546,16 @@ public:
             // The original delta units the scrollBegin and scrollUpdates
             // were sent as.
             ScrollUnits deltaUnits;
+            // If true, this event comes after a non-inertial gesture
+            // scroll sequence; OSX has unique phases for normal and

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

Is this correct? If this is true, the event comes after an inertial gesture
scroll sequence, right?

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

correct; I had copied the text from the begin which isn't really correct.

+            // momentum scroll events. Should always be false for touch based
+            // input as it generates GestureFlingStart instead.
+            bool inertial;
+            // True if the begin was generated when phase information is available
+            // for touchpad and for every scroll update we are generating a GSB/GSU/GSE
+            // tuple. This is a temporary variable that forces the non-latching behavior
+            // until crbug.com/526463 is fully implemented.

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

I'd mention that these are only needed to match with the synthesized GSB events
that we use to force hit testing to avoid scroll latching.

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

Done.

+            bool synthetic;
         } scrollEnd;
 
         struct {