Bug 1612106 - Use NIGHTLY build flag to distinguish the writable/readonly Animation.timeline. r=birtles,bzbarsky

We only support writable Animation.timeline attribute on nightly
build, so use nightly build flag in WebIDL and Animation.h/cpp.
For the test expectations, we also use this build flag to skip them or
mark them as expected fail if necessary.

Differential Revision: https://phabricator.services.mozilla.com/D63432

dom/animation/Animation.cpp

@@ -189,6 +189,10 @@ void Animation::SetEffectNoUpdate(AnimationEffect* aEffect) {
 }
 
 void Animation::SetTimeline(AnimationTimeline* aTimeline) {
+#ifndef NIGHTLY_BUILD
+  MOZ_ASSERT_UNREACHABLE(
+      "Animation.timeline setter is supported only on nightly");
+#endif
   SetTimelineNoUpdate(aTimeline);
   PostUpdate();
 }