Simplify feature engagement tracker API.

Instead of splitting out preconditions, trigger-events and whether
a feature has been used, they are now all just events. This
simplifies the API surface quite a bit.

Whether a feature has been used will now happen by marking a
particular event as the one that signifies that a feature has been
used. This will be configured per in-product help feature.

The check for whether the in-product help should be trigger is
still a similar method that returns a boolean.

BUG=706309

Change-Id: I309f95ebc417d078e9e671b271b2d50e7e8f0fb2
Reviewed-on: https://chromium-review.googlesource.com/466586
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#461587}
8 files changed