WPT: Ensure src attribute changes trigger preparation
See this HTML Standard PR thread:
whatwg/html#10188 (comment).
R=domenic
Bug: N/A
Change-Id: I31065f76e324f71d2b4ae52813b3b37d3b4467c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5809759
Reviewed-by: Domenic Denicola domenic@chromium.org
Commit-Queue: Dominic Farolino dom@chromium.org
Cr-Commit-Position: refs/heads/main@{#1346661}