Convert USE_PROPRIETARY_CODECS to a buildflag header.

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index 53fe23fc72c3688f1874e5cb2438d4fc4b57fc74..564b45a8b823e59e81be03d1239e093f55bdfcbd 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -35,6 +35,9 @@ declare_args() {
 
   # Enables proprietary codecs and demuxers; e.g. H264, AAC, MP3, and MP4.
   # We always build Google Chrome and Chromecast with proprietary codecs.
+  #
+  # Note: this flag is used by WebRTC which is DEPSed into Chrome. Moving it
+  # out of //build will require using the build_overrides directory.
   proprietary_codecs = is_chrome_branded || is_chromecast
 
   # Variable safe_browsing is used to control the build time configuration for