Revert "cc: Remove glFinish call during cc shutdown."

This reverts commit b148b478.

This change was causing hangs on Android. Was able to repro locally
and bisect with high confidence. I agree that this logically should
be removable, but for some reason isn't. We should re-visit in a
future milestone.

TBR=danakj,piman

Bug: 846709
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1029fa57bc3bfb919121535fc2b70f6884e88a15
Reviewed-on: https://chromium-review.googlesource.com/1154198Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578909}

cc/trees/layer_tree_host_impl.cc

@@ -3102,6 +3102,11 @@ void LayerTreeHostImpl::ReleaseLayerTreeFrameSink() {
   resource_pool_ = nullptr;
   ClearUIResources();
 
+  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);