H.264: Default flags and pulling in openh264 and ffmpeg.

Defining use_third_party_h264 directly, and indirectly defining use_openh264 (from third_party/openh264) and ffmpeg_branding (from third_party/ffmpeg).
These will be used in a follow-up CL that adds an encoder and decoder implementation.
The flags are added in this CL so that they can be used by trybots/waterfall bots in GN without "Build argument had no effect" errors. Equivalent GYP changes are also added.

BUG=468365

Review URL: https://codereview.webrtc.org/1575913003

Cr-Commit-Position: refs/heads/master@{#11204}

setup_links.py

@@ -40,6 +40,7 @@ DIRECTORIES = [
   'third_party/colorama',
   'third_party/drmemory',
   'third_party/expat',
+  'third_party/ffmpeg',
   'third_party/instrumented_libraries',
   'third_party/jsoncpp',
   'third_party/libc++-static',
@@ -53,6 +54,7 @@ DIRECTORIES = [
   'third_party/lss',
   'third_party/nss',
   'third_party/ocmock',
+  'third_party/openh264',
   'third_party/openmax_dl',
   'third_party/opus',
   'third_party/proguard',