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

content/browser/gpu/gpu_main_thread_factory.h

+// 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.
+
+#ifndef CONTENT_BROWSER_GPU_GPU_MAIN_THREAD_FACTORY_H_
+#define CONTENT_BROWSER_GPU_GPU_MAIN_THREAD_FACTORY_H_
+
+#include "content/common/content_export.h"
+
+namespace base {
+class Thread;
+}
+
+namespace gpu {
+struct GpuPreferences;
+}
+
+namespace content {
+class InProcessChildThreadParams;
+
+typedef base::Thread* (*GpuMainThreadFactoryFunction)(
+    const InProcessChildThreadParams&,
+    const gpu::GpuPreferences&);
+
+CONTENT_EXPORT void RegisterGpuMainThreadFactory(
+    GpuMainThreadFactoryFunction create);
+
+GpuMainThreadFactoryFunction GetGpuMainThreadFactory();
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_GPU_GPU_MAIN_THREAD_FACTORY_H_