Bug 1077402 - Lock monitor when accessing overscrolling transform. r=kats

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2459,6 +2459,7 @@ bool AsyncPanZoomController::UpdateAnimation(const TimeStamp& aSampleTime,
 }
 
 Matrix4x4 AsyncPanZoomController::GetOverscrollTransform() const {
+  ReentrantMonitorAutoEnter lock(mMonitor);
   if (!IsOverscrolled()) {
     return Matrix4x4();
   }