[ios] Refactor //ios/web find_in_page.js search

Changes highlightWord nomenclature to findString, and removes logic that
searches through all child frames. Refactor pumpSearch to not highlight
first result. findString and pumpSearch also only returns numerical value
of matches since decision which frame to highligh/scroll to is done by
manager.

Bug: 896867, 895529
Change-Id: Ib71ffaffda96e20a0e0721df0085476e998ae017
Reviewed-on: https://chromium-review.googlesource.com/c/1387045
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622662}
3 files changed