Fix APZ not starting and updating wheel animations within an APZC lock. (bug 1152011 part 1, r=botond)

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1538,6 +1538,11 @@ nsEventStatus AsyncPanZoomController::OnScrollWheel(const ScrollWheelInput& aEve
     }
 
     case ScrollWheelInput::SCROLLMODE_SMOOTH: {
+      // The lock must be held across the entire update operation, so the
+      // compositor doesn't end the animation before we get a chance to
+      // update it.
+      ReentrantMonitorAutoEnter lock(mMonitor);
+
       if (mState != WHEEL_SCROLL) {
         CancelAnimation();
         SetState(WHEEL_SCROLL);