Renderers should obtain browser InterfaceProvider by connecting to browser

content/public/browser/content_browser_client.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_CONTENT_BROWSER_CLIENT_H_
 #define CONTENT_PUBLIC_BROWSER_CONTENT_BROWSER_CLIENT_H_
 
 #include <stddef.h>
 
 #include <map>
@@ skipping 626 matching lines @@
   virtual bool IsPluginAllowedToUseDevChannelAPIs(
       BrowserContext* browser_context,
       const GURL& url);
 
   // Generate a Shell user-id for the supplied browser context. Defaults to
   // returning a random GUID.
   virtual std::string GetShellUserIdForBrowserContext(
       BrowserContext* browser_context);
 
   // Allows to register browser Mojo interfaces exposed through the
-  // RenderProcessHost.
+  // RenderProcessHost. Note that interface factory callbacks added to
+  // |registry| will by default be run immediately on the IO thread, unless a
+  // task runner is provided.
   virtual void ExposeInterfacesToRenderer(
       shell::InterfaceRegistry* registry,
       RenderProcessHost* render_process_host) {}
 
   // Called when RenderFrameHostImpl connects to the Media service. Expose
   // interfaces to the service using |registry|.
   virtual void ExposeInterfacesToMediaService(
       shell::InterfaceRegistry* registry,
       RenderFrameHost* render_frame_host) {}
 
@@ skipping 117 matching lines @@
   // Allows an embedder to provide its own ExternalVideoSurfaceContainer
   // implementation.  Return nullptr to disable external surface video.
   virtual ExternalVideoSurfaceContainer*
   OverrideCreateExternalVideoSurfaceContainer(WebContents* web_contents);
 #endif
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_CONTENT_BROWSER_CLIENT_H_