Add enable_hls_demuxer GN arg

This CL adds the `enable_hls_demuxer` arg to media_options.gni, which
defaults to false. There is currently no code landed behind this arg,
but having this arg checked in simplifies the process of protoyping. For
a preview of code which will be landed in future CLs behind this arg,
please see the prototype HLS demuxer:
https://chromium-review.googlesource.com/c/chromium/src/+/3096950

Bug: 1266991
Change-Id: Ic7c232750b7973f6e199c561da0053a72195b164
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3262414
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#938658}

media/BUILD.gn

@@ -34,6 +34,7 @@ buildflag_header("media_buildflags") {
     "ENABLE_PLATFORM_HEVC_DECODING=$enable_platform_hevc_decoding",
     "ENABLE_PLATFORM_ENCRYPTED_HEVC=$enable_platform_encrypted_hevc",
     "ENABLE_HLS_SAMPLE_AES=$enable_hls_sample_aes",
+    "ENABLE_HLS_DEMUXER=$enable_hls_demuxer",
     "ENABLE_LIBGAV1_DECODER=$enable_libgav1_decoder",
     "ENABLE_LIBRARY_CDMS=$enable_library_cdms",
     "ENABLE_LIBVPX=$media_use_libvpx",

media/media_options.gni

@@ -74,6 +74,9 @@ declare_args() {
   # Enable browser managed persistent metadata storage for EME persistent
   # session and persistent usage record session.
   enable_media_drm_storage = is_android || is_chromecast
+
+  # Enable HLS manifest parser and demuxer.
+  enable_hls_demuxer = false
 }
 
 declare_args() {
@@ -98,6 +101,10 @@ assert(
     !enable_hls_sample_aes || enable_mse_mpeg2ts_stream_parser,
     "enable_mse_mpeg2ts_stream_parser=true is required for enable_hls_sample_aes=true.")
 
+assert(
+    !enable_hls_demuxer || enable_mse_mpeg2ts_stream_parser,
+    "enable_mse_mpeg2ts_stream_parser=true is required for enable_hls_demuxer=true.")
+
 assert(!enable_platform_hevc || proprietary_codecs,
        "proprietary_codecs=true is required for enable_platform_hevc=true.")