Convert USE_PROPRIETARY_CODECS to a buildflag header.

build/config/features.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # These flags are effectively global. Your feature flag should go near the
 # code it controls. Most of these items are here now because they control
@@ skipping 17 matching lines @@
                 current_cpu != "mipsel" && !(is_linux && target_cpu == "arm64")
 
   # Non-SFI is not yet supported on mipsel
   enable_nacl_nonsfi = current_cpu != "mipsel"
 
   # Enables the Media Router.
   enable_media_router = !is_ios && !is_chromecast
 
   # 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
   # safe browsing feature. Safe browsing can be compiled in 3 different levels:
   # 0 disables it, 1 enables it fully, and 2 enables mobile protection via an
   # external API.
   if (is_ios || is_chromecast) {
     safe_browsing_mode = 0
   } else if (is_android) {
     safe_browsing_mode = 2
@@ skipping 31 matching lines @@
 
 # Chrome OS: whether to also build the upcoming version of
 # ChromeVox, which can then be enabled via a command-line switch.
 enable_chromevox_next = false
 #
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # See comment at the top.