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Implements logic for FindInPageSearch option to search and highlight matches.
Searching for non-ascii not currently supported.

Bug: 896867
Change-Id: I362d3cebacfced1537742fbbab9ad323d6edeafd
Reviewed-on: https://chromium-review.googlesource.com/c/1407665Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
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@{#626108}