When looking up the break index for a given source position, default to
the last break index if there is neither a precise match nor a breakable
position after the source position (in which case we still pick the
first candidate).

Fixed: chromium:1222065
Bug: chromium:901819, chromium:782461, chromium:1222060
Change-Id: I10d6a086b2d5fadc9e6dca0c49ed4187eb0359ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2972917
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Kim-Anh Tran <kimanh@chromium.org>
Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75284}