Do not access AudioWorklet::IsReady() from the audio thread

AudioWorklet::IsReady() checks the state of the worklet object and
must be accessed from the main thread.

OfflineAudioDestinationHandler::DoOfflineRendering() violates the rule
and this CL fixes GetRenderingThread() method to check the assigned
worklet thread instead.

Bug: 804908
Change-Id: I1ff1f779a3bb75d1715f521a2656a7c6c8c51a9f
Reviewed-on: https://chromium-review.googlesource.com/882121Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532571}

third_party/WebKit/Source/modules/webaudio/OfflineAudioDestinationNode.cpp

@@ -360,18 +360,17 @@ bool OfflineAudioDestinationHandler::RenderIfNotSuspended(
 WebThread* OfflineAudioDestinationHandler::GetRenderingThread() {
   DCHECK(IsInitialized());
 
-  // Use Experimental AudioWorkletThread only when AudioWorklet is enabled, and
-  // the worklet thread and the global scope are ready.
-  if (Context()->audioWorklet() && Context()->audioWorklet()->IsReady()) {
-    DCHECK(!render_thread_ && worklet_backing_thread_);
+  if (worklet_backing_thread_) {
     return worklet_backing_thread_;
   }
 
-  DCHECK(render_thread_ && !worklet_backing_thread_);
+  DCHECK(render_thread_);
   return render_thread_.get();
 }
 
 void OfflineAudioDestinationHandler::RestartRendering() {
+  DCHECK(IsMainThread());
+
   // If the worklet thread is not assigned yet, that means the context has
   // started without a valid WorkletGlobalScope. Assign the worklet thread,
   // and it will be picked up when the GetRenderingThread() is called next.