The ReplacementStringBuilder had a tricky API that required manually
growing the backing store if needed. This saves a few int additions
and comparisons, but is probably not worth the potential for errors.
This CL changes to automatic growth that is invisible to the user.

Bug: chromium:933521
Change-Id: I7d2404a48fe47ccce9af919910c06a9eca598120
Reviewed-on: https://chromium-review.googlesource.com/c/1477748Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59726}