Fixed failing DCHECK in GpuVideoDecodeAccelerator destructor.

BUG=

Review URL: https://codereview.chromium.org/314293004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275513 0039d316-1c4b-4281-b951-d872f2087c98

content/common/gpu/media/gpu_video_decode_accelerator.cc

@@ -459,6 +459,7 @@ void GpuVideoDecodeAccelerator::OnWillDestroyStub() {
   stub_->channel()->RemoveRoute(host_route_id_);
   stub_->RemoveDestructionObserver(this);
 
+  video_decode_accelerator_.reset();
   delete this;
 }