Post callback asynchronously from the MediaStream constructor

This CL makes sure initialized callback from the MediaStream constructor
is called asynchronously as it was causing a JavaScript crash
potentially caused by the fact that the MediaStream was not completely
registered with oilpan.

Bug: 1114398
Change-Id: If6058b183bdd07fa747d810efe0f6abed4914e49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352800Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#797649}

third_party/blink/renderer/modules/mediastream/media_stream.cc

@@ -148,7 +148,10 @@ MediaStream::MediaStream(ExecutionContext* context,
   }
 
   if (number_of_video_tracks == 0) {
-    std::move(media_stream_initialized_callback_).Run(this);
+    context->GetTaskRunner(TaskType::kInternalMedia)
+        ->PostTask(FROM_HERE,
+                   WTF::Bind(std::move(media_stream_initialized_callback_),
+                             WrapPersistent(this)));
   }
 }
 

third_party/blink/renderer/modules/mediastream/media_stream_track.cc

@@ -256,7 +256,8 @@ MediaStreamTrack::MediaStreamTrack(ExecutionContext* context,
     image_capture_ = MakeGarbageCollected<ImageCapture>(
         context, this, pan_tilt_zoom_allowed, std::move(callback));
   } else {
-    std::move(callback).Run();
+    execution_context_->GetTaskRunner(TaskType::kInternalMedia)
+        ->PostTask(FROM_HERE, std::move(callback));
   }
 }