Add base:: to string16 in content/.

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 7f37194857a94407a5cdcb5261d72c9edeffe83b..c7a7010121e53887da278ff78c465178ab6f13de 100644
--- a/content/public/browser/web_contents_delegate.h
+++ b/content/public/browser/web_contents_delegate.h
@@ -455,8 +455,8 @@ class CONTENT_EXPORT WebContentsDelegate {
   // has failed. There can only be one message per |web_contents| at a time.
   virtual void ShowValidationMessage(WebContents* web_contents,
                                      const gfx::Rect& anchor_in_root_view,
-                                     const string16& main_text,
-                                     const string16& sub_text) {}
+                                     const base::string16& main_text,
+                                     const base::string16& sub_text) {}
 
   // Notification that the delegate should hide any showing form validation
   // message.