Bug 1585523 - Use NSAnimationContext instead of CATransaction when triggering off-main-thread composites. r=mattwoodrow

When called on a non-main thread, [NSAnimationContext beginGrouping] calls [CATransaction activateBackground:].
There's a chance that doing so might help with some of the crashes we're seeing during [CATransaction commit].

Differential Revision: https://phabricator.services.mozilla.com/D47854

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

widget/cocoa/nsChildView.mm

@@ -2031,10 +2031,10 @@ void nsChildView::PostRender(WidgetRenderingContext* aContext) {
       compositingState->mNativeLayerChangesPending = true;
     } else {
       // Force a CoreAnimation layer tree update from this thread.
-      [CATransaction begin];
+      [NSAnimationContext beginGrouping];
       mNativeLayerRoot->ApplyChanges();
       compositingState->mNativeLayerChangesPending = false;
-      [CATransaction commit];
+      [NSAnimationContext endGrouping];
     }
   } else {
     UniquePtr<GLManager> manager(GLManager::CreateGLManager(aContext->mLayerManager));