Remove more platform-specific GN flags

flags.gn

@@ -18,22 +18,16 @@ enable_swiftshader=false
 enable_widevine=true
 exclude_unwind_tables=true
 fatal_linker_warnings=false
-ffmpeg_branding="Chrome"
 fieldtrial_testing_like_official_build=true
 google_api_key=""
 google_default_client_id=""
 google_default_client_secret=""
-is_clang=true
 is_debug=false
 is_official_build=true
 optimize_webui=false
-proprietary_codecs=true
 safe_browsing_mode=0
 symbol_level=0
 treat_warnings_as_errors=false
-use_gnome_keyring=false
-use_jumbo_build=true
 use_official_google_api_keys=false
 use_ozone=false
-use_sysroot=false
 use_unofficial_version_number=false