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}
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index cbfe8c0..32ac627 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -136,6 +136,18 @@
   deps = [
     "../../system_wrappers",
   ]
+
+  if (use_third_party_h264) {
+    # Dependency added so that variables use_openh264 and ffmpeg_branding are
+    # recognized build arguments (avoid "Build argument has no effect" error).
+    # The variables and dependencies will be used for real as soon as
+    # https://codereview.webrtc.org/1306813009/ lands. In the meantime, the
+    # build arguments are to be used by waterfall/trybots.
+    deps += [
+      "//third_party/ffmpeg:ffmpeg",
+      "//third_party/openh264:encoder",
+    ]
+  }
 }
 
 # TODO(tkchin): Source set for webrtc_h264_video_toolbox. Currently not