Convert USE_PROPRIETARY_CODECS to a buildflag header.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 3aab0a04970f66f4e334c9beb5929ddb5b38e84d..00d3e442974ab4391e4bbf210844f0be72b4dfab 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -7,6 +7,14 @@ import("//build/config/features.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
 declare_args() {
+  # Enables proprietary codecs and demuxers; e.g. H264, AAC, MP3, and MP4.
+  # We always build Google Chrome and Chromecast with proprietary codecs.
+  proprietary_codecs = is_chrome_branded || is_chromecast
+}
+
+# Must be a separate block since some of these depend on the proprietary_codecs
+# flag.
+declare_args() {
   # Allows distributions to link pulseaudio directly (DT_NEEDED) instead of
   # using dlopen. This helps with automated detection of ABI mismatches and
   # prevents silent errors.