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This moves the PageAlmostIdle and SiteCharacteristicsDatabase features to their
own experiment so they can easily be shipped to Beta only.

Change-Id: I56c65e4ffb8b389b502a8c03d661ca2c26a66abe
Reviewed-on: https://chromium-review.googlesource.com/1168107Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582394}