[Gap Decorations]: Correct minor error in ink overflow calculation

This CL addresses a minor error in the previous implementation related
to ink overflow calculation. The mistake was using `inline_size` instead
of `inline_start`, which resulted in the invalidation rect being larger
than needed. This was identified while developing the updated ink
overflow code for optimized gap geometry. To keep things clear and
focused, I've opted to have both changes in separate CL rather than
combined as one CL.

Bug: 357648037
Change-Id: Ibfac582ecdecab0ac2a00978557a433d467570d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6940889
Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Sam Davis Omekara <samomekarajr@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1514943}