Immediately dismiss tracking_only help UI.

The feature engagement system previously expected their client to always
dismiss after ShouldTriggerHelpUI(). However, this wasn't reasonable for
tracking only features, because false is always returned by
ShouldTriggerHelpUI() and the client has no idea when they should
dismiss.

To work around this the TrackerImpl now always dismisses tracking only
features to reset their condition validator. This isn't a perfect
solution, as real help UI will almost never be dismissed in 0 seconds,
but it should be a good enough approximation.

Bug: 1188679
Change-Id: I179b1fb4a97c752b5d9ed0a91fbb4c5e9d915def
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773179


Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864689}