Update from https://crrev.com/309415

cc/resources/tile_task_worker_pool.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/resources/tile_task_worker_pool.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "base/lazy_instance.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/simple_thread.h"
 #include "cc/base/scoped_ptr_deque.h"
 #include "cc/resources/raster_source.h"
 #include "skia/ext/refptr.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkSurface.h"
 
 namespace cc {
 namespace {
 
+base::ThreadPriority g_worker_thread_priority = base::kThreadPriority_Normal;
+
 class TileTaskGraphRunner : public TaskGraphRunner,
                             public base::DelegateSimpleThread::Delegate {
  public:
   TileTaskGraphRunner() {
     size_t num_threads = TileTaskWorkerPool::GetNumWorkerThreads();
     while (workers_.size() < num_threads) {
       scoped_ptr<base::DelegateSimpleThread> worker =
           make_scoped_ptr(new base::DelegateSimpleThread(
               this, base::StringPrintf(
                         "CompositorTileWorker%u",
                         static_cast<unsigned>(workers_.size() + 1)).c_str()));
       worker->Start();
-#if defined(OS_ANDROID) || defined(OS_LINUX)
-      worker->SetThreadPriority(base::kThreadPriority_Background);
-#endif
+      worker->SetThreadPriority(g_worker_thread_priority);
       workers_.push_back(worker.Pass());
     }
   }
 
   ~TileTaskGraphRunner() override { NOTREACHED(); }
 
  private:
   // Overridden from base::DelegateSimpleThread::Delegate:
   void Run() override { TaskGraphRunner::Run(); }
 
@@ skipping 66 matching lines @@
 
 // static
 int TileTaskWorkerPool::GetNumWorkerThreads() {
   if (!g_num_worker_threads)
     g_num_worker_threads = kDefaultNumWorkerThreads;
 
   return g_num_worker_threads;
 }
 
 // static
+void TileTaskWorkerPool::SetWorkerThreadPriority(
+    base::ThreadPriority priority) {
+  g_worker_thread_priority = priority;
+}
+
+// static
 TaskGraphRunner* TileTaskWorkerPool::GetTaskGraphRunner() {
   return g_task_graph_runner.Pointer();
 }
 
 // static
 scoped_refptr<TileTask> TileTaskWorkerPool::CreateTaskSetFinishedTask(
     base::SequencedTaskRunner* task_runner,
     const base::Closure& on_task_set_finished_callback) {
   return make_scoped_refptr(
       new TaskSetFinishedTaskImpl(task_runner, on_task_set_finished_callback));
@@ skipping 121 matching lines @@
   // TODO(kaanb): The GL pipeline assumes a 4-byte alignment for the
   // bitmap data. There will be no need to call SkAlign4 once crbug.com/293728
   // is fixed.
   const size_t dst_row_bytes = SkAlign4(dst_info.minRowBytes());
   DCHECK_EQ(0u, dst_row_bytes % 4);
   bool success = canvas->readPixels(dst_info, memory, dst_row_bytes, 0, 0);
   DCHECK_EQ(true, success);
 }
 
 }  // namespace cc