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This method the last time at which the state of a LifecycleUnit
changed. It will be used to determine if a LifeycleUnit has been
unfrozen long enough and should be frozen again.

Bug: 775644
Change-Id: I2ba04b9ed4085767e67108cd46901d876efc2217
Reviewed-on: https://chromium-review.googlesource.com/1134029
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575067}