Enable ImprovedSourceRetargeting by default

This is already resolved in whatwg, see discussion here:
whatwg/html#11345

Fixed: 420639769
Change-Id: I82a543bc6f93f14b487da4373492462cbdb33867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6853857
Commit-Queue: Joey Arhar <[email protected]>
Reviewed-by: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1511147}

Author: josepharhar