Add field to monitor last visited time for each search engine

1. Add the last_visited field in template_url_data to monitor last
visited time of each search engine.
2. The last_visited will be updated to current time each time the OpenSearch
description document is detected.
3. For now, last_visited will not be synced if the template_url is
existed.

BUG=348360

Review-Url: https://codereview.chromium.org/2498053002
Cr-Commit-Position: refs/heads/master@{#436079}
17 files changed