Reset GPUVideoEncodeAccelerator::encoder_ when destroying

I broke this in https://src.chromium.org/viewvc/chrome?revision=273860.
The DCHECK is to make sure that the proper destruction sequence is
happening. Reset encoder_ so the DCHECK is valid again.

Already fixed for the decoder here:
https://src.chromium.org/viewvc/chrome?view=rev&revision=275513

BUG=386170

Review URL: https://codereview.chromium.org/331293006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278132 0039d316-1c4b-4281-b951-d872f2087c98

content/common/gpu/media/gpu_video_encode_accelerator.cc

@@ -149,7 +149,7 @@ void GpuVideoEncodeAccelerator::OnWillDestroyStub() {
   DCHECK(stub_);
   stub_->channel()->RemoveRoute(host_route_id_);
   stub_->RemoveDestructionObserver(this);
-
+  encoder_.reset();
   delete this;
 }