AnimationWorklet: Check for threaded animations before constructing a WorkletAnimation

This avoids hitting the DCHECK in WorkletAnimation::WorkletAnimation in
the case where --disable-threaded-animations is specified.

Bug: 825127
Change-Id: I5e2eb0684e472d5a88f40ad07f19f73c0118b627
Reviewed-on: https://chromium-review.googlesource.com/980234Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545824}

third_party/WebKit/Source/modules/animationworklet/WorkletAnimation.cpp

@@ -155,6 +155,13 @@ WorkletAnimation* WorkletAnimation::Create(
     ExceptionState& exception_state) {
   DCHECK(IsMainThread());
 
+  if (!Platform::Current()->IsThreadedAnimationEnabled()) {
+    exception_state.ThrowDOMException(
+        kInvalidStateError,
+        "AnimationWorklet requires threaded animations to be enabled");
+    return nullptr;
+  }
+
   HeapVector<Member<KeyframeEffect>> keyframe_effects;
   String error_string;
   if (!ConvertAnimationEffects(effects, keyframe_effects, error_string)) {