Gstreamer backend reports that `audio/webm` mime type is not supported

[tharkum]

After implementation of supported selection of media element source from source children (PR servo/servo#39717) there was found that GStreamer backend is reporting that audio/webm mime type is not supported while it is not true.

OK (audio)
<audio src="any-audio-content.oga">

OK (audio + source without type)
<audio><source src="any-audio-content.oga"></audio>

FAIL (audio + source with type)
<audio><source src="any-audio-content.oga" type="audio/webm"></audio>

See https://html.spec.whatwg.org/multipage/media.html#loading-the-media-resource:attr-source-type

Need to figure out reason why it happens (see backends/gstreamer/registry_scanner.rs)

Some associated WPT tests (media-src) on which it is reproduced with test expectation changes (with vs without):

• /content-security-policy/media-src/media-src-7_2_2.sub.html
• /content-security-policy/media-src/media-src-blocked.sub.html

NOTE: Some audio are encoded with Vorbis and both is decoding by Gstreamer eventually...
Vorbis: media/sound_5.oga

On local PC gst-inspect-1.0 reports the following plugins (ubuntu22.04)

gst-inspect-1.0 | grep webm
matroska:  webmmux: WebM muxer
typefindfunctions: video/x-matroska: mkv, mka, mk3d, webm

gst-inspect-1.0 | grep opus
libav:  avdec_cllc: libav Canopus Lossless Codec decoder
libav:  avdec_hq_hqa: libav Canopus HQ/HQA decoder
libav:  avdec_hqx: libav Canopus HQX decoder
libav:  avdec_opus: libav Opus decoder
libav:  avenc_opus: libav Opus encoder
libav:  avmux_opus: libav Ogg Opus muxer
opus:  opusdec: Opus audio decoder
opus:  opusenc: Opus audio encoder
opusparse:  opusparse: Opus audio parser
rtp:  rtpopusdepay: RTP Opus packet depayloader
rtp:  rtpopuspay: RTP Opus payloader

gst-inspect-1.0 | grep vorbis
rtp:  rtpvorbisdepay: RTP Vorbis depayloader
rtp:  rtpvorbispay: RTP Vorbis payloader
typefindfunctions: audio/x-vorbis: no extensions
vorbis:  vorbisdec: Vorbis audio decoder
vorbis:  vorbisenc: Vorbis audio encoder
vorbis:  vorbisparse: VorbisParse
vorbis:  vorbistag: VorbisTag