If the vsync provider is being replaced, make sure the animator expects no...

If the vsync provider is being replaced, make sure the animator expects no more signals from the old registration. (#2954)

sky/shell/ui/animator.cc

@@ -106,6 +106,10 @@ void Animator::RequestFrame() {
 
 void Animator::set_vsync_provider(vsync::VSyncProviderPtr vsync_provider) {
   vsync_provider_ = vsync_provider.Pass();
+
+  // We may be waiting on a VSync signal from the old VSync provider.
+  pending_frame_semaphore_.Signal();
+
   RequestFrame();
 }