This metric was being sent in LockStateController::OnRealPowerTimeout,
which is also used by e.g. the tray shut down button.

Add ShutdownReason enum with user actions that can trigger shutdown.

Pass ShutdownReason from user event handlers through to
ShutdownController where it is used to send metrics. Currently only
ShutdownReason::POWER_BUTTON is handled here as other metrics are
reported in other locations.

Remove `shutdown_after_lock` param from
LockStateController::StatLockAnimation in favor of new
StartLockThenShutdownAnimation method.

BUG=chromium:694039
TEST=manually verified correct shutdown reasons

Change-Id: I62bddff668a9134d472308d9f5e06283d4b94d1a
Reviewed-on: https://chromium-review.googlesource.com/543905


Commit-Queue: Lann Martin <lannm@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483750}