H264: Thread-safe InitializeFFmpeg. Flag to control if InitializeFFmpeg should be called.

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 124 matching lines @@
 
     # Enable this to use HW H.264 encoder/decoder on iOS/Mac PeerConnections.
     # Enabling this may break interop with Android clients that support H264.
     'use_objc_h264%': 0,
 
     # Enable this to build OpenH264 encoder/FFmpeg decoder. This is supported on
     # all platforms except iOS. Because FFmpeg can be built with/without H.264
     # support, |ffmpeg_branding| has to separately be set to a value that
     # includes H.264, for example "Chrome". If FFmpeg is built without H.264,
     # compilation succeeds but |H264DecoderImpl| fails to initialize.
+    # See also: |rtc_initialize_ffmpeg|.
     # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
     # http://www.openh264.org, https://www.ffmpeg.org/
     'rtc_use_h264%': 0,
 
     'conditions': [
+      # FFmpeg must be initialized for |H264DecoderImpl| to work. This can be
+      # done by WebRTC during |H264DecoderImpl::InitDecode| or externally.
+      # FFmpeg must only be initialized once. Projects that initialize FFmpeg
+      # externally, such as Chromium, must turn this flag off so that WebRTC
+      # does not also initialize.
+      ['build_with_chromium==0', {
+        'rtc_initialize_ffmpeg%': 1,
+      }, {
+        'rtc_initialize_ffmpeg%': 0,
+      }],
+
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require
         # pulse audio.
         'include_pulse_audio%': 0,
 
         # Exclude internal ADM since Chromium uses its own IO handling.
         'include_internal_audio_device%': 0,
 
         # Remove tests for Chromium to avoid slowing down GYP generation.
         'include_tests%': 0,
@@ skipping 302 matching lines @@
           # of a more specific macro.
           'defines': [
             'WEBRTC_POSIX',
           ],
         }],
       ],
     },
   }, # target_defaults
 }