[Android] Cancel a fling if necessary before programmatic scrolls

The compositor expects a fling cancellation event if the active fling is
interrupted by a new scroll sequence. For programmatic scrolls via
ContentViewCore.scrollTo, this cancellation event is never sent,
potentially confusing the compositor if a fling is active. Send the
cancellation event if a fling is potentially active.

Review URL: https://codereview.chromium.org/727963003

Cr-Commit-Position: refs/heads/master@{#304724}

content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java

@@ -1688,6 +1688,10 @@ public void scrollTo(int xPix, int yPix) {
         final float dyPix = yPix - yCurrentPix;
         if (dxPix != 0 || dyPix != 0) {
             long time = SystemClock.uptimeMillis();
+            // It's a very real (and valid) possibility that a fling may still
+            // be active when programatically scrolling. Cancelling the fling in
+            // such cases ensures a consistent gesture event stream.
+            if (mPotentiallyActiveFlingCount > 0) nativeFlingCancel(mNativeContentViewCore, time);
             nativeScrollBegin(mNativeContentViewCore, time,
                     xCurrentPix, yCurrentPix, -dxPix, -dyPix);
             nativeScrollBy(mNativeContentViewCore,