PerformanceTestTrait script/stylesheet size counting needs to handle external URLs

LGTM

longwave → closed merge request !12518

I can think of slightly neater ways of writing this code but as this is an edge case mostly only used by @catch then let's just ship it and move on :)

Committed 57f7daa and pushed to 11.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.