Make TextFinder Scoping use Idle Tasks

Currently scoping (searching for all find matches) is done in 100ms
chunks and uses timers. We want to make it use idle tasks instead to
ensure this doesn't block important tasks.

Bug: 868227
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Iaa9f40580fe5c69af398ae43a339d06305767503
Reviewed-on: https://chromium-review.googlesource.com/1152710
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584034}
17 files changed