Add functionality to get default media device IDs from user preferences.

content/browser/frame_host/render_frame_host_delegate.h

 // Copyright 2013 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_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_
 #define CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 140 matching lines @@
   virtual void RequestMediaAccessPermission(
       const MediaStreamRequest& request,
       const MediaResponseCallback& callback);
 
   // Checks if we have permission to access the microphone or camera. Note that
   // this does not query the user. |type| must be MEDIA_DEVICE_AUDIO_CAPTURE
   // or MEDIA_DEVICE_VIDEO_CAPTURE.
   virtual bool CheckMediaAccessPermission(const GURL& security_origin,
                                           MediaStreamType type);
 
+  // Returns the ID of the default device for the given media device |type|.
+  // If the returned value is an empty string, it means that there is no
+  // default device for the given |type|.
+  virtual std::string GetDefaultMediaDeviceId(MediaStreamType type);

[clamy, 2017/01/16 12:48:09]

It seems to me that there are no instances of this function being used in the
patch. I'd like to avoid having a patch land with non-exercised code as much as
possible :). If you plan on using it in another CL, could you consider folding
the two of them together? Thanks!

[Guido Urdaneta, 2017/01/18 15:03:59]

I added a test that exercises most of the code, although it still does not
exercise the top-level function
RenderFrameHostDelegate::GetDefaultMediaDeviceId.
The reason is that it cannot know about the chrome namespace and viceversa, and
the value for the default devices can only be set in the chrome namespace.
The test uses WebContentsDelegate::GetDefaultMediaDeviceId which is accessible
in chrome.

[clamy, 2017/01/18 15:15:49]

I understand, but why do you need to add this particular function that is not
used? How do you intend to use it afterwards? Without example of usage it's a
bit hard for me to review whether it makes sense for it to be here or not.

+
   // Get the accessibility mode for the WebContents that owns this frame.
   virtual AccessibilityMode GetAccessibilityMode() const;
 
   // Called when accessibility events or location changes are received
   // from a render frame, when the accessibility mode has the
   // ACCESSIBILITY_MODE_FLAG_WEB_CONTENTS flag set.
   virtual void AccessibilityEventReceived(
       const std::vector<AXEventNotificationDetails>& details) {}
   virtual void AccessibilityLocationChangesReceived(
       const std::vector<AXLocationChangeNotificationDetails>& details) {}
@@ skipping 98 matching lines @@
                                  const gfx::Rect& initial_rect,
                                  bool user_gesture) {}
 
  protected:
   virtual ~RenderFrameHostDelegate() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_