Add VTTRegion and part of VTTCue DOM interfaces

[iulianR]

Hello! In this PR I implemented the VTTRegion DOM interface and part of VTTCue (#22312). Before continuing I thought it's maybe a good idea to first request a review or even merge what I did up to this point, as I might get stuck on the next part. I have a slight idea of what needs to be done (I assume it's hooking the GStreamer parser somewhere in GetCueAsHTML()), but a short outline or some more instructions would help a lot.

I updated test expectations by first running:

./mach test-wpt tests/wpt/web-platform-tests/webvtt/api --log-raw servo.log --pref dom.webvtt.enabled=true

then

./mach update-wpt servo.log

Thanks!

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/webidls/VTTCue.webidl, components/script/dom/vttregion.rs, components/script/dom/mod.rs, components/script/dom/texttrackcue.rs, components/script/dom/webidls/VTTRegion.webidl and 1 more
• @KiChjang: components/script/dom/webidls/VTTCue.webidl, components/script/dom/vttregion.rs, components/script/dom/mod.rs, components/script/dom/texttrackcue.rs, components/script/dom/webidls/VTTRegion.webidl and 1 more

[jdm]

This base implementation looks good so far!

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 7293cf1 has been approved by jdm

[bors-servo]

⌛ Testing commit 7293cf1 with merge 95f3d46...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: jdm
Pushing 95f3d46 to master...