Fix a null-dereference in the MediaStreamTrack constructor

Bug: 1128167
Change-Id: Ic79945517ee854a18788f08efe1686707086354b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418399
Commit-Queue: Palak Agarwal <agpalak@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808802}

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

@@ -259,8 +259,12 @@ MediaStreamTrack::MediaStreamTrack(ExecutionContext* context,
     image_capture_ = MakeGarbageCollected<ImageCapture>(
         context, this, pan_tilt_zoom_allowed, std::move(callback));
   } else {
+    if (execution_context_) {
       execution_context_->GetTaskRunner(TaskType::kInternalMedia)
           ->PostTask(FROM_HERE, std::move(callback));
+    } else {
+      std::move(callback).Run();
+    }
   }
 
   // Note that both 'live' and 'muted' correspond to a 'live' ready state in the