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

content/public/browser/web_contents_delegate.cc

Index: content/public/browser/web_contents_delegate.cc
diff --git a/content/public/browser/web_contents_delegate.cc b/content/public/browser/web_contents_delegate.cc
index 01c9c0897ea087977474a91e4ce3c0f758710243..8a384730d04e59f5cab5b457d4241574ac6e9028 100644
--- a/content/public/browser/web_contents_delegate.cc
+++ b/content/public/browser/web_contents_delegate.cc
@@ -201,6 +201,12 @@ bool WebContentsDelegate::CheckMediaAccessPermission(
   return false;
 }
 
+std::string WebContentsDelegate::GetDefaultMediaDeviceID(
+    WebContents* web_contents,
+    MediaStreamType type) {
+  return std::string();
+}
+
 #if defined(OS_ANDROID)
 void WebContentsDelegate::RequestMediaDecodePermission(
     WebContents* web_contents,