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Adding a utility class to handle the logic of calculating if enough time
has passed. The last backgrounded time is already tracked in
ChromeTabbedActivity::onStopWithNative().

Experiment name is enable-tab-switcher-on-return. See `chrome://flags`

Available options are 30 or 60 minutes.

R=yusufo@chromium.org

Bug: 898685
Change-Id: I916eb86b55525f06335d27cec84de78b1e366e98
Reviewed-on: https://chromium-review.googlesource.com/c/1384658Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Matt Simmons <mattsimmons@google.com>
Cr-Commit-Position: refs/heads/master@{#618309}