Add gpu_main_thread_factory.h to expose less of content's guts.

content/browser/gpu/gpu_main_thread_factory.cc

+// Copyright 2017 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 "content/browser/gpu/gpu_main_thread_factory.h"
+
+namespace content {
+namespace {
+
+GpuMainThreadFactoryFunction g_gpu_main_thread_factory = nullptr;
+
+}  // namespace
+
+void RegisterGpuMainThreadFactory(GpuMainThreadFactoryFunction create) {
+  g_gpu_main_thread_factory = create;
+}
+
+GpuMainThreadFactoryFunction GetGpuMainThreadFactory() {
+  return g_gpu_main_thread_factory;
+}
+
+}  // namespace content