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

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index a77420c070cb103d4c42fd179a592b0a3272542d..066cee67a841643ab20674148f75a2ccb652b4a5 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -142,10 +142,17 @@
     # 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_skip_ffmpeg_init|.
     # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
     # http://www.openh264.org, https://www.ffmpeg.org/
     'rtc_use_h264%': 0,
 
+    # 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 set this flag so that WebRTC does not also initialize.
+    'rtc_skip_ffmpeg_init%': '<(build_with_chromium)',
+
     'conditions': [
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require