Add WebView API for smoothScroll

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index aa82c34ad3f228b6c1ba68d89694b3f43f71dc00..40677d4548c443218d961515ad5444122346e9bd 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -617,6 +617,10 @@ public:
     // with |velocity| and already scrolled |cumulativeScroll| pixels.
     virtual WebGestureCurve* createFlingAnimationCurve(WebGestureDevice deviceSource, const WebFloatPoint& velocity, const WebSize& cumulativeScroll) { return nullptr; }
 
+
+    virtual WebGestureCurve* createSmoothScrollAnimationCurve(
+        WebGestureDevice deviceSource, float startX, float startY, float dx, float dy, int64_t duration) { return 0; }
+
     // WebRTC ----------------------------------------------------------
 
     // Creates an WebRTCPeerConnectionHandler for RTCPeerConnection.