Do not resume OfflineAudioContext when it is cleared

Previously OfflineAudioContext::resumeContext() method did not check
if the context is cleared by ExecutionContext::ContextDestroyed().
Such case is possible when the audio context is a part of a detached
iframe.

This CL changes the check so we can verify if the context's resources
is still available. Otherwise, we can reject the resume promise
resolver.

Test: Locally confirmed ASAN does not crash with the repro case.
Bug: 1048373
Change-Id: I96a601dcf63963525d95cfc5089fd4a3b0176687
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042409
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739085}

third_party/blink/renderer/modules/webaudio/offline_audio_context.cc

@@ -329,8 +329,9 @@ ScriptPromise OfflineAudioContext::resumeContext(ScriptState* script_state) {
     return promise;
   }
 
-  // If the context is in a closed state, reject the promise.
-  if (ContextState() == AudioContextState::kClosed) {
+  // If the context is in a closed state or it really is closed (cleared),
+  // reject the promise.
+  if (IsContextClosed()) {
     resolver->Reject(MakeGarbageCollected<DOMException>(
         DOMExceptionCode::kInvalidStateError,
         "cannot resume a closed offline context"));