CopyChars takes a count parameter, not an end parameter, so we can save
some subtractions by passing in the count to WriteToFlat. Most of the
time the start,end arguments into WriteToFlat are 0,length anyway, so
this doesn't change a lot of places.

Change-Id: I9587c7afce529218a16b728c0477b87569df8e21
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3157947Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76813}