Remove Flush in LayerTreeResourceProvider::PrepareSendToParent

It is not needed for correctness, and it has overhead. Work is generally
already flushed in TileManager::FlushAndIssueSignals.

Bug: 836497
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I53156defe40fe0818ca05f8cfc2cf8d5faf99e02
Reviewed-on: https://chromium-review.googlesource.com/1035919Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554888}

cc/resources/layer_tree_resource_provider.cc

@@ -161,11 +161,6 @@ void LayerTreeResourceProvider::PrepareSendToParent(
     }
   }
 
-  // TODO(piman): See if this FlushPendingWork is needed.
-  // https://crbug.com/836497
-  if (compositor_context_provider_)
-    compositor_context_provider_->ContextSupport()->FlushPendingWork();
-
   if (!unverified_sync_tokens.empty()) {
     DCHECK(settings_.delegated_sync_points_required);
     DCHECK(gl);