Bug 1571253 - Call mProfilerScreenshotGrabber.NotifyEmptyFrame() in the main place where no-op composites are noticed. r=mattwoodrow

There's another call inside Render() but that's usually too late.

Depends on D40558

Differential Revision: https://phabricator.services.mozilla.com/D40559

--HG--
extra : moz-landing-system : lando

gfx/layers/composite/LayerManagerComposite.cpp

@@ -591,6 +591,7 @@ void LayerManagerComposite::UpdateAndRender() {
   if (invalid.IsEmpty() && !mWindowOverlayChanged) {
     // Composition requested, but nothing has changed. Don't do any work.
     mClonedLayerTreeProperties = LayerProperties::CloneFrom(GetRoot());
+    mProfilerScreenshotGrabber.NotifyEmptyFrame();
     return;
   }