Prevent SET_SCROLL_OFFSET spam from clogging tubes

Bug: 3393571
 Limit the event queue to a single pending SET_SCROLL_OFFSET. The
 problem on slashdot.org was that the event thread was being flooded
 with SET_SCROLL_OFFSET messages faster than it could process them,
 causing the handler to get jammed up.

Change-Id: Ia4a90a030959d0b3769067791089cdb261ae403a

core/java/android/webkit/WebView.java

@@ -2506,6 +2506,7 @@ public class WebView extends AbsoluteLayout
         // Rect.equals() checks for null input.
         if (!rect.equals(mLastVisibleRectSent)) {
             Point pos = new Point(rect.left, rect.top);
+            mWebViewCore.removeMessages(EventHub.SET_SCROLL_OFFSET);
             mWebViewCore.sendMessage(EventHub.SET_SCROLL_OFFSET,
                     nativeMoveGeneration(), mUserScroll ? 1 : 0, pos);
             mLastVisibleRectSent = rect;