[DevTools] Explicitly add Tethering domain to browser protocol.

chrome/browser/devtools/devtools_protocol_constants.gyp

 # Copyright 2014 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.
 
 {
   'targets': [
     {
       'target_name': 'devtools_protocol_constants',
       'type': 'none',
       'actions': [
         {
           'action_name': 'devtools_protocol_constants',
           'variables': {
             'blink_protocol': '../../../third_party/WebKit/Source/devtools/protocol.json',
+            'browser_protocol': '../../../content/browser/devtools/browser_protocol.json',
             'generator': '../../../content/public/browser/devtools_protocol_constants_generator.py',
             'package': 'chrome'
           },
           'inputs': [
             '<(blink_protocol)',
+            '<(browser_protocol)',
             '<(generator)',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/<(package)/browser/devtools/devtools_protocol_constants.cc',
             '<(SHARED_INTERMEDIATE_DIR)/<(package)/browser/devtools/devtools_protocol_constants.h'
           ],
           'action':[
             'python',
             '<(generator)',
             '<(package)',
             '<(SHARED_INTERMEDIATE_DIR)/<(package)/browser/devtools/devtools_protocol_constants.cc',
             '<(SHARED_INTERMEDIATE_DIR)/<(package)/browser/devtools/devtools_protocol_constants.h',
             '<(blink_protocol)',
+            '<(browser_protocol)',
           ],
-          'message': 'Generating DevTools protocol constants from <(blink_protocol)'
+          'message': 'Generating DevTools protocol constants from <(blink_protocol) and <(browser_protocol)'
         }
       ],
     },
   ],
 }