Add WebView API for smoothScroll

public/web/WebView.h

Index: public/web/WebView.h
diff --git a/public/web/WebView.h b/public/web/WebView.h
index 40462065090ca19d4128f3192a6eb13260c3ea37..af9c1b7abc7630f069d756f7916a0a47b3d5dc5d 100644
--- a/public/web/WebView.h
+++ b/public/web/WebView.h
@@ -176,6 +176,10 @@ public:
     // viewport space. Returns true if an animation was started.
     virtual bool scrollFocusedNodeIntoRect(const WebRect&) { return false; }
 
+
+    // Smooth scroll the root layer to |targetX|, |targetY| in |durationMs|.
+    virtual void smoothScroll(int targetX, int targetY, long durationMs);

[jdduke (slow), 2015/07/27 15:49:02]

I guess this handles all of pageDown/pageUp/requestChildRectOnScreen? Just
wondering if targetX/targetY is more convenient than deltaX/deltaY?

[hush (inactive), 2015/07/28 00:16:06]

yes. This handles all three.
I use target position instead of delta location because that's what
startPageScaleAnimation needs.

Actually, there is no need for "deltaX" and "deltaY" to exist in the first
place. "AwScrollOffsetManager#animateScrollTo" simply passes the target position
to content view core.

+
     // Advance the focus of the WebView forward to the next element or to the
     // previous element in the tab sequence (if reverse is true).
     virtual void advanceFocus(bool reverse) { }