RC: Record Chrome usage time when a LifecycleUnit is hidden.

With this CL, the total Chrome usage time measured by UsageClock is
recorded in a LifecycleUnit when it is hidden. In an upcoming CL, this
will be used to discard a LifecycleUnit after a given amount of Chrome
usage time has elapsed since it was hidden.

We also record the wall time when a LifecycleUnit is hidden, because
freezing occurs when a LifeycleUnit has been hidden for a given amount
of wall clock time, not Chrome usage time.

Bug: 775644
Change-Id: I76d504e9bbf3eb86ded1203e4e117086faf93277
Reviewed-on: https://chromium-review.googlesource.com/1133593
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574712}
18 files changed