Test context_->GetCommandBufferProxy() before following it blindly.

Apparently it's possible to have the context WeakPtr not be cleared yet but have
the underlying proxy already be gone.

BUG=141880


Review URL: https://chromiumcodereview.appspot.com/10830265

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151193 0039d316-1c4b-4281-b951-d872f2087c98

content/renderer/media/renderer_gpu_video_decoder_factories.cc

@@ -67,7 +67,7 @@ void RendererGpuVideoDecoderFactories::AsyncCreateVideoDecodeAccelerator(
       media::VideoDecodeAccelerator** vda,
       base::WaitableEvent* waiter) {
   DCHECK_EQ(MessageLoop::current(), message_loop_);
-  if (context_) {
+  if (context_ && context_->GetCommandBufferProxy()) {
     *vda = gpu_channel_host_->CreateVideoDecoder(
         context_->GetCommandBufferProxy()->GetRouteID(),
         profile, client);