[NTP] Allow blacklisting and opt-out of suggestions

Provide several new embeddedSearch APIs for blacklisting NTP search
suggestions and opting out:

window.chrome.embeddedSearch.newTabPage.blacklistSearchSuggestion(task_version, task_id)
window.chrome.embeddedSearch.newTabPage.blacklistSearchSuggestionWithHash(task_version, task_id, hash)
window.chrome.embeddedSearch.newTabPage.optOutOfSearchSuggestions()

Store blacklist and opt-out in user prefs. Append blacklist to
requests for suggestions as the url param 'vtgb'.

Bug: 904565
Change-Id: I308a57af9e61808d78bfaf606bdfe03bdd7a7acb
Reviewed-on: https://chromium-review.googlesource.com/c/1384526
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624157}
22 files changed