Convert USE_PROPRIETARY_CODECS to a buildflag header.

Convert the preprocessor define to a buildflag
//header defined in //media. The build arg is kept in //build/config/features.gni due to a dependency on WebRTC (described in a comment).

Reland of https://codereview.chromium.org/2572883002 with no changes. I believe the cause of the previous revert was a now-fixed GN race condition.

TBR=xhwang@chromium.org

Review-Url: https://codereview.chromium.org/2643333002
Cr-Original-Commit-Position: refs/heads/master@{#445226}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 68c7753dd291fbf573dc67c258284440f5aad6ff

config/BUILD.gn

@@ -136,9 +136,6 @@ config("feature_flags") {
   if (enable_wayland_server) {
     defines += [ "ENABLE_WAYLAND_SERVER=1" ]
   }
-  if (proprietary_codecs) {
-    defines += [ "USE_PROPRIETARY_CODECS" ]
-  }
   if (safe_browsing_mode == 1) {
     defines += [ "FULL_SAFE_BROWSING" ]
     defines += [ "SAFE_BROWSING_CSD" ]

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