[text-box-trim] Make ClearanceAfterLine() optional
Following up crrev.com/c/5561849, this patch supports when
ClearanceAfterLine() exists but 0, by changing it to
std::optional<LayoutUnit>.
Bug: 40254880
Change-Id: I407d6cf418b3e4765881ef454ca7f38e5d66338e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5564483
Reviewed-by: Kent Tamura tkent@chromium.org
Commit-Queue: Koji Ishii kojii@chromium.org
Cr-Commit-Position: refs/heads/main@{#1306284}