script: Claim blob before loading <video> poster frame

[servo-wpt-sync]

Refer to servo/servo#43746 for a description of the problem. This change ensures that video posters can be loaded from blob URLs, even if the URL is revoked right after.

Testing: This change adds a test
Reviewed in servo/servo#44035

[wpt-pr-bot]

The review process for this patch is being conducted in the Servo project.

[servo-wpt-sync]

⛔ The downstream PR has merged (servo/servo#44035), but these changes could not be merged properly. Please address any CI issues and try to merge manually.

[jdm]

@simonwuelker Looks like the test might have intermittent results in other browsers, which could be due to implementation bugs like the one we fixed in Servo. I will ask in matrix how to proceed.

[simonwuelker]

Maybe the test removes test-wait too early. I'll check later.

The problem is probably using oncanplaythrough. The existing video-poster-shown-auto-preload.html test (which I based my test on) waits for that event to delay comparison for reftests, but I think that only works because the poster fetch itself already delays the load event.

I'm unsure how to make sure that the screenshot is taken after the poster is loaded... Since in our case the load event fires long before the blob URL is created, so its not delayed.