Add a menu item to content_shell to open devtools to make it more discoverable.

content/shell/resource.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.
 
 //{{NO_DEPENDENCIES}}
 // Microsoft Visual C++ generated include file.
 // Used by test_shell.rc
 //
 
 
 #define IDR_MAINFRAME             128
 #define IDM_EXIT                  105
 #define IDM_CLOSE_WINDOW          106
 #define IDM_NEW_WINDOW            107
+#define IDM_SHOW_DEVELOPER_TOOLS  108
 #define IDC_CONTENTSHELL          109
 #define IDD_ALERT                 130
 #define IDD_CONFIRM               131
 #define IDD_PROMPT                132
 #define IDC_NAV_BACK              1001
 #define IDC_NAV_FORWARD           1002
 #define IDC_NAV_RELOAD            1003
 #define IDC_NAV_STOP              1004
 #define IDC_PROMPTEDIT            1005
 #define IDC_DIALOGTEXT            1006
 
 #ifndef IDC_STATIC
 #define IDC_STATIC                -1
 #endif
 // Next default values for new objects
 //
 #ifdef APSTUDIO_INVOKED
 #ifndef APSTUDIO_READONLY_SYMBOLS
 
 #define _APS_NO_MFC               130
 #define _APS_NEXT_RESOURCE_VALUE  129
 #define _APS_NEXT_COMMAND_VALUE   32771
 #define _APS_NEXT_CONTROL_VALUE   1000
 #define _APS_NEXT_SYMED_VALUE     117
 #endif
 #endif