Add build_protobuf variable.

webrtc/build/common.gypi

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # This file contains common settings for building WebRTC components.
 
@@ skipping 73 matching lines @@
     'enable_protobuf%': 1,
 
     # Disable these to not build components which can be externally provided.
     'build_expat%': 1,
     'build_json%': 1,
     'build_libjpeg%': 1,
     'build_libvpx%': 1,
     'build_libyuv%': 1,
     'build_openmax_dl%': 1,
     'build_opus%': 1,
+    'build_protobuf%': 1,
     'build_ssl%': 1,
 
     # Disable by default
     'have_dbus_glib%': 0,
 
     # Make it possible to provide custom locations for some libraries.
     'libvpx_dir%': '<(DEPTH)/third_party/libvpx_new',
     'libyuv_dir%': '<(DEPTH)/third_party/libyuv',
     'opus_dir%': '<(opus_dir)',
 
@@ skipping 346 matching lines @@
           # of a more specific macro.
           'defines': [
             'WEBRTC_POSIX',
           ],
         }],
       ],
     },
   }, # target_defaults
 }