Clear out pending SwapBufferComplete count when activating the compositor.

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 9c9901c4515c1cf94ed5ea937950c411bd7f54af..4fa87b921be2fe81b353e2285cbfb98684270b67 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1160,6 +1160,10 @@ void RenderWidget::didActivateCompositor(int input_handler_identifier) {
   is_accelerated_compositing_active_ = true;
   Send(new ViewHostMsg_DidActivateAcceleratedCompositing(
       routing_id_, is_accelerated_compositing_active_));
+
+  // Ignore any swapbuffer complete callbacks corresponding
+  // to swapbuffer calls made before the compositor was activated.
+  num_swapbuffers_complete_pending_ = 0;
 }
 
 void RenderWidget::didDeactivateCompositor() {