vr/gpu: BrowserGpuChannelHostFactory should be called on the same thread.

content/public/browser/android/compositor.h

 // Copyright (c) 2012 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_PUBLIC_BROWSER_ANDROID_COMPOSITOR_H_
 #define CONTENT_PUBLIC_BROWSER_ANDROID_COMPOSITOR_H_
 
 #include "base/callback.h"
 #include "cc/resources/ui_resource_bitmap.h"
 #include "content/common/content_export.h"
@@ skipping 27 matching lines @@
 // An interface to the browser-side compositor.
 class CONTENT_EXPORT Compositor {
  public:
   virtual ~Compositor() {}
 
   // Performs the global initialization needed before any compositor
   // instance can be used. This should be called only once.
   static void Initialize();
 
   // Creates a GL context for the provided |handle|. If a null handle is passed,
-  // an offscreen context is created.
+  // an offscreen context is created. This must be called on the UI thread.
   using ContextProviderCallback =
       base::Callback<void(scoped_refptr<cc::ContextProvider>)>;
   static void CreateContextProvider(
       gpu::SurfaceHandle handle,
       gpu::gles2::ContextCreationAttribHelper attributes,
       gpu::SharedMemoryLimits shared_memory_limits,
       ContextProviderCallback callback);
 
   // Creates and returns a compositor instance.  |root_window| needs to outlive
   // the compositor as it manages callbacks on the compositor.
@@ skipping 22 matching lines @@
   // Returns the resource manager associated with the compositor.
   virtual ui::ResourceManager& GetResourceManager() = 0;
 
  protected:
   Compositor() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_ANDROID_COMPOSITOR_H_