Make Editor::FindString use flat-tree FindRangeOfString

We're going to remove DOM tree version of FindRangeOfString soon.

Bug: 868227
Change-Id: I42a9d2a417efde646ee20b5d2825a24765d3369e
Reviewed-on: https://chromium-review.googlesource.com/c/1451500
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628654}
1 file changed