Set WEBRTC_USE_H264 in common_config

Bug: none
Change-Id: I3dce8fdc409c88cdd771ea57eca3ea375e6e82c9
Reviewed-on: https://webrtc-review.googlesource.com/c/121948
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26603}
diff --git a/BUILD.gn b/BUILD.gn
index 96210fd..1cb920e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -242,6 +242,10 @@
     defines += [ "ENABLE_EXTERNAL_AUTH" ]
   }
 
+  if (rtc_use_h264) {
+    defines += [ "WEBRTC_USE_H264" ]
+  }
+
   if (build_with_chromium) {
     defines += [
       # NOTICE: Since common_inherited_config is used in public_configs for our