cc: Use correct message loop proxy in BlockingTaskRunner

The compositor makes it possible to customize the message loop proxy
both on the main and impl threads. However BlockingTaskRunner just uses
base::MessageLoopProxy::current() directly and thus overrides this
choice. Using the correct message loop proxy is important because
clients of BlockingTaskRunner have an implicit dependency on the
ordering of tasks posted to BlockingTaskRunner vs. all other tasks
posted to the task runner for that same thread.

This patch fixes the problem by removing the thread-local
BlockingTaskRunner::current() instance in favor of an explicitly
constructed instance which is given a specific SingleThreadTaskRunner
to use. The resource provider is changed to pass a Proxy-owned
blocking task runner to the clients which need one.

BUG=391005
TEST=1. Apply https://codereview.chromium.org/363383002
     2. out/Debug/chrome --disable-impl-side-painting --ignore-gpu-blacklist tools/perf/page_sets/tough_scheduling_cases/raf_canvas.html

Review URL: https://codereview.chromium.org/485043003

Cr-Commit-Position: refs/heads/master@{#293353}

1 files changed, 6 insertions, 3 deletions

diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 1ccbdb9d..0e8c81c 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -192,7 +192,8 @@ void SingleThreadProxy::DoCommit(const BeginFrameArgs& begin_frame_args) {
     // This CapturePostTasks should be destroyed before CommitComplete() is
     // called since that goes out to the embedder, and we want the embedder
     // to receive its callbacks before that.
-    BlockingTaskRunner::CapturePostTasks blocked;
+    BlockingTaskRunner::CapturePostTasks blocked(
+        blocking_main_thread_task_runner());
 
     layer_tree_host_impl_->BeginCommit();
 
@@ -308,7 +309,8 @@ void SingleThreadProxy::Stop() {
     DebugScopedSetMainThreadBlocked main_thread_blocked(this);
     DebugScopedSetImplThread impl(this);
 
-    BlockingTaskRunner::CapturePostTasks blocked;
+    BlockingTaskRunner::CapturePostTasks blocked(
+        blocking_main_thread_task_runner());
     layer_tree_host_->DeleteContentsTexturesOnImplThread(
         layer_tree_host_impl_->resource_provider());
     scheduler_on_impl_thread_.reset();
@@ -534,7 +536,8 @@ DrawResult SingleThreadProxy::DoComposite(base::TimeTicks frame_begin_time,
     // the swap buffers will execute first.
     DebugScopedSetMainThreadBlocked main_thread_blocked(this);
 
-    BlockingTaskRunner::CapturePostTasks blocked;
+    BlockingTaskRunner::CapturePostTasks blocked(
+        blocking_main_thread_task_runner());
     layer_tree_host_impl_->SwapBuffers(*frame);
   }
   DidCommitAndDrawFrame();