bindings: Do not get if timeline is missing (1/N)

Bindings is going to update IDL dictionaries to be strict about their
members' existence.  After the update, accessing a getter on a missing
dictionary member will crash.

This CL fixes the potential bug in Animatable.animate().


Bug: 839389
Change-Id: I305daff7130a2a26b5a7142f67fe7920910cead3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226483Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774571}

third_party/blink/renderer/core/animation/animatable.cc

@@ -77,24 +77,18 @@ Animation* Animatable::animate(
     return element->GetDocument().Timeline().Play(effect);
 
   Animation* animation;
-  AnimationTimeline* timeline =
-      options.GetAsKeyframeAnimationOptions()->timeline();
-
-  bool timeline_is_undefined =
-      !options.GetAsKeyframeAnimationOptions()->hasTimeline();
-  bool timeline_is_null = !timeline_is_undefined && !timeline;
-
-  if (timeline_is_undefined) {
+  const KeyframeAnimationOptions* options_dict =
+      options.GetAsKeyframeAnimationOptions();
+  if (!options_dict->hasTimeline()) {
     animation = element->GetDocument().Timeline().Play(effect);
-  } else if (timeline_is_null) {
-    animation = Animation::Create(element->GetExecutionContext(), effect,
-                                  timeline, exception_state);
-  } else {
-    DCHECK(timeline);
+  } else if (AnimationTimeline* timeline = options_dict->timeline()) {
     animation = timeline->Play(effect);
+  } else {
+    animation = Animation::Create(element->GetExecutionContext(), effect,
+                                  nullptr, exception_state);
   }
 
-  animation->setId(options.GetAsKeyframeAnimationOptions()->id());
+  animation->setId(options_dict->id());
   return animation;
 }