Allow FindBuffer to find text bounded by a range

This change allows FindBuffer to search for find-in-page matches that
are located inside a range of [start_position, end_position). This will
allow FindBuffer to be used by Editor::FindRangeOfString in the future,
removing that function's usage of TextIterator.

Bug: 868227
Change-Id: Id5b8c04ffa1995b9ee614f58a90901a5cf4075a7
Reviewed-on: https://chromium-review.googlesource.com/c/1424677
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626040}
4 files changed