Move Find-in-page scoping to FindTaskController

This CL moves find-in-page scoping methods and idle task to a new
class, FindTaskController and use the term "finding" instead of
"scoping" there. FindTaskController is responsible for managing
IdleFindTask, and notifying TextFinder of matches etc, so that
TextFinder can manage markers, tickmark, notifying frame etc.

Bug: 868227
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I20701ec0542acb40ace471891a490612e73d700b
Reviewed-on: https://chromium-review.googlesource.com/c/1278074
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599605}
6 files changed