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

content/browser/frame_host/render_frame_host_delegate.h

Index: content/browser/frame_host/render_frame_host_delegate.h
diff --git a/content/browser/frame_host/render_frame_host_delegate.h b/content/browser/frame_host/render_frame_host_delegate.h
index 4f3178036cda45d6dee1f0b9e6f4d0bbd2672ad9..1ab65e1c393373e1f7df2539c8654d0deb108f06 100644
--- a/content/browser/frame_host/render_frame_host_delegate.h
+++ b/content/browser/frame_host/render_frame_host_delegate.h
@@ -158,6 +158,11 @@ class CONTENT_EXPORT RenderFrameHostDelegate {
   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;