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

content/public/browser/android/compositor.h

Index: content/public/browser/android/compositor.h
diff --git a/content/public/browser/android/compositor.h b/content/public/browser/android/compositor.h
index 02f188645bed7a4a1c6cf18a6628d34795bebc07..42213059118ad7c901c7b2978536e0806f588536 100644
--- a/content/public/browser/android/compositor.h
+++ b/content/public/browser/android/compositor.h
@@ -45,7 +45,7 @@ class CONTENT_EXPORT Compositor {
   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(