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

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index f784ab1830f64625f0dc75a967cc54804b810d5e..886b8995433635f8c6f21c353197114e17bef154 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -96,9 +96,17 @@ declare_args() {
   # 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 = false
+
+  # 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.
+  rtc_initialize_ffmpeg = !build_with_chromium
 }
 
 # A second declare_args block, so that declarations within it can