[break-iterator] Move the runtime flag check out of loop

This patch moves the runtime flag check out of the per-
character loop, by using templates.

This patch has no behavior changes.

Bug: 41485013
Change-Id: Ic337b6005c992095c3f14834b137509d95ea6025
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5467480
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1290307}