Add WebView API for smoothScroll

public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 476 matching lines @@
             bool preventPropagation;
             bool inertial;
         } scrollUpdate;
 
         struct {
             float velocityX;
             float velocityY;
             // 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 {
             // If set to true, don't treat flingCancel
             // as a part of fling boost events sequence.
             bool preventBoosting;
         } flingCancel;
 
         struct {
             bool zoomDisabled;
@@ skipping 47 matching lines @@
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif