Fix such that compositor_frame_sink is not null for OnContextLost.

These calls were accidently misordered. |compositor_frame_sink_| needs to
stick around for the completion of resource_provider_->OnContextLost(),
as it uses the |compositor_frame_sink_| for DidDeleteSharedBitmap.

TBR=junov@chromium.org

Bug: 864657
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I24ace6f8766a2c4d9c77b359d61c4c4531b9b14e
Reviewed-on: https://chromium-review.googlesource.com/1142827Reviewed-by: CJ DiMeglio <lethalantidote@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: CJ DiMeglio <lethalantidote@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576370}

third_party/blink/renderer/platform/graphics/video_frame_submitter.cc

@@ -301,10 +301,11 @@ void VideoFrameSubmitter::OnContextLost() {
   }
   waiting_for_compositor_ack_ = false;
 
+  resource_provider_->OnContextLost();
+
   // |compositor_frame_sink_| should be reset last.
   compositor_frame_sink_.reset();
 
-  resource_provider_->OnContextLost();
   context_provider_callback_.Run(
       base::BindOnce(&VideoFrameSubmitter::OnReceivedContextProvider,
                      weak_ptr_factory_.GetWeakPtr()));