The protector is useful for follow-up optimizations on string iterator.
Tests are also added.

Change-Id: I416037c742628c4d4d3b878d0df727a9ae7162f7
Reviewed-on: https://chromium-review.googlesource.com/1251122Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Hai Dang <dhai@google.com>
Cr-Commit-Position: refs/heads/master@{#56315}