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Moving all the highlighting logic from DOM Agent to Overlay agent.

Bug: 775348
Change-Id: I5ae4fc7bc178566147f306d9ac5c04834e683c0c
Reviewed-on: https://chromium-review.googlesource.com/964704Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Yi Xu <yiyix@chromium.org>
Commit-Queue: Illia Martyniuk <illiam@google.com>
Cr-Commit-Position: refs/heads/master@{#544168}