Linux: add a tool and code to make use_system_ffmpeg option more compatible

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 723 matching lines @@
     'enable_message_center%': '<(enable_message_center)',
     'use_default_render_theme%': '<(use_default_render_theme)',
     'enable_web_intents%': '<(enable_web_intents)',
     'enable_settings_app%': '<(enable_settings_app)',
     '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)',
     'enable_managed_users%': '<(enable_managed_users)',
 
+    # Use system ffmpeg instead of bundled one.
+    'use_system_ffmpeg%': 0,
+
     # Use system mesa instead of bundled one.
     'use_system_mesa%': 0,
 
     # Use system nspr instead of the bundled one.
     'use_system_nspr%': 0,
 
     # Use system protobuf instead of bundled one.
     'use_system_protobuf%': 0,
 
     # Use system yasm instead of bundled one.
@@ skipping 3289 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',
   },
 }