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Reland of https://codereview.chromium.org/2614443002, which was reverted due to static initializer.

end_date is a replacement for expiry_date, which is deprecated by this change. The difference is that end_date is acts as a normal filter, and causes study to not be created when the date is past. expiry_date caused the study to be created, and forced to the default group.

This change introduces a generic default group name, which is used for the field trial API when a study doesn't specify one.

BUG=677976

Review-Url: https://codereview.chromium.org/2612243003
Cr-Commit-Position: refs/heads/master@{#441976}