Add explicit base namespace to string16 users.

chrome/common/print_messages.h

 // Copyright (c) 2012 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.
 
 // IPC messages for printing.
 // Multiply-included message file, hence no include guard.
 
 #include <string>
 #include <vector>
 
@@ skipping 27 matching lines @@
   int desired_dpi;
   int document_cookie;
   bool selection_only;
   bool supports_alpha_blend;
   int32 preview_ui_id;
   int preview_request_id;
   bool is_first_request;
   blink::WebPrintScalingOption print_scaling_option;
   bool print_to_pdf;
   bool display_header_footer;
-  string16 title;
-  string16 url;
+  base::string16 title;
+  base::string16 url;
   bool should_print_backgrounds;
 };
 
 struct PrintMsg_PrintPages_Params {
   PrintMsg_PrintPages_Params();
   ~PrintMsg_PrintPages_Params();
 
   // Resets the members of the struct to 0.
   void Reset();
 
@@ skipping 371 matching lines @@
 IPC_SYNC_MESSAGE_ROUTED0_0(PrintHostMsg_SetupScriptedPrintPreview)
 
 // Tell the browser to show the print preview, when the document is sufficiently
 // loaded such that the renderer can determine whether it is modifiable or not.
 IPC_MESSAGE_ROUTED1(PrintHostMsg_ShowScriptedPrintPreview,
                     bool /* is_modifiable */)
 
 // Notify the browser that the PDF in the initiator renderer has disabled print
 // scaling option.
 IPC_MESSAGE_ROUTED0(PrintHostMsg_PrintPreviewScalingDisabled)