Add WebView API for smoothScroll

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index 476cfd7442e91bcadf1109994ae7a49dd0000932..e2329f7ae6733fd92853f9c4f582e23485ee603c 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -494,6 +494,12 @@ public:
             // If true, this event will skip hit testing to find a scroll
             // target and instead just scroll the viewport.
             bool targetViewport;
+
+            bool isSmoothScroll;
+            // The following 3 fields are valid when isSmoothScroll == true.
+            float dx;

[jdduke (slow), 2015/07/23 15:19:02]

Do we need dx/dy if we have a velocity + duration?

I guess smooth scroll can have a slightly variable velocity at the
beginning/end, but if we're sharing the same event type I'd prefer not to have
"clashing" fields.

+            float dy;
+            long durationMs;
         } flingStart;
 
         struct {