Linux: add an option to build with system protobuf.

build/common.gypi

 # 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 646 matching lines @@
     'wix_path%': '<(wix_path)',
     'android_upstream_bringup%': '<(android_upstream_bringup)',
     'use_libjpeg_turbo%': '<(use_libjpeg_turbo)',
     'use_system_libjpeg%': '<(use_system_libjpeg)',
     'android_build_type%': '<(android_build_type)',
     'use_official_google_api_keys%': '<(use_official_google_api_keys)',
     'google_api_key%': '<(google_api_key)',
     'google_default_client_id%': '<(google_default_client_id)',
     'google_default_client_secret%': '<(google_default_client_secret)',
 
+    # Use system protobuf instead of bundled one.
+    'use_system_protobuf%': 0,
+
     # Use system yasm instead of bundled one.
     'use_system_yasm%': 0,
 
     # Default to enabled PIE; this is important for ASLR but we may need to be
     # able to turn it off for various reasons.
     'linux_disable_pie%': 0,
 
     # The release channel that this build targets. This is used to restrict
     # channel-specific build options, like which installer packages to create.
     # The default is 'all', which does no channel-specific filtering.
@@ skipping 2995 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }