cc: Remove glFinish call during cc shutdown.

From piman:
I'm almost 100% positive we don't need this any more.
At the origin, when the renderer was rendering directly to a child
HWND/XWindow, and pre-ForceCompositingMode, we needed this to avoid
flashing when going out of compositing mode, but that constraint is
long long gone. On the Display compositor, we also need a synchronous
IPC on command buffer destruction, but that still exists regardless
of this.

R=piman@chromium.org

Bug: 826886
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I15ed61c1f661ab2f615cae6f69f48eeb51d8e7ee
Reviewed-on: https://chromium-review.googlesource.com/1062730
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559541}

cc/trees/layer_tree_host_impl.cc

@@ -2892,11 +2892,6 @@ void LayerTreeHostImpl::ReleaseLayerTreeFrameSink() {
   ClearUIResources();
   resource_provider_ = nullptr;
 
-  if (layer_tree_frame_sink_->context_provider()) {
-    auto* gl = layer_tree_frame_sink_->context_provider()->ContextGL();
-    gl->Finish();
-  }
-
   // Release any context visibility before we destroy the LayerTreeFrameSink.
   SetContextVisibility(false);