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

content/public/browser/web_contents_delegate.h

Index: content/public/browser/web_contents_delegate.h
diff --git a/content/public/browser/web_contents_delegate.h b/content/public/browser/web_contents_delegate.h
index 6456878bd456cf878f75660e0a3887625e674ca2..793d00f72ed4499a80b46142a2aa379534931d34 100644
--- a/content/public/browser/web_contents_delegate.h
+++ b/content/public/browser/web_contents_delegate.h
@@ -479,6 +479,12 @@ class CONTENT_EXPORT WebContentsDelegate {
                                           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(WebContents* web_contents,
+                                              MediaStreamType type);
+
 #if defined(OS_ANDROID)
   // Asks permission to decode media stream. After permission is determined,
   // |callback| will be called with the result.