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This method returns a unique id representing the LifecycleUnit. It
is required to support the chrome://discards/ page.

Bug: 775644
Change-Id: I5975e625337708cc57beca0a5d551e6352c51b6a
Reviewed-on: https://chromium-review.googlesource.com/789250
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521363}