Change LifecycleUnit::GetLastVisibilityChangeTime() to GetLastVisibleTime().

All existing callers really want to know when the LifecycleUnit was
last visible, not when its visibility last changed (they are not
interested in transitions between visible and hidden).

Change-Id: Ie57d8c3219d69cdb5ede938544a434063ebe9e04
Reviewed-on: https://chromium-review.googlesource.com/1069451
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561358}
8 files changed