Bug 1574521 - Make sure updateLayer is only called when the CoreAnimation pref is set. r=jrmuizel a=Aryx

updateLayer expects some objects to be non-null which are only initialized when the pref is set.
But in builds that were compiled with the 10.14 SDK, all NSViews are layer-backed by default, so
wantspdateLayer gets called, and returning YES from that unconditionally will cause updateLayer
to be called.

Differential Revision: https://phabricator.services.mozilla.com/D42333

--HG--
extra : amend_source : a7a1f7ecba9418e3bf5464bca90ba655002bc637

widget/cocoa/nsChildView.mm

@@ -6240,7 +6240,7 @@ nsresult nsChildView::GetSelectionAsPlaintext(nsAString& aResult) {
 }
 
 - (BOOL)wantsUpdateLayer {
-  return YES;
+  return StaticPrefs::gfx_core_animation_enabled_AtStartup();
 }
 
 - (void)updateLayer {