andrewlewis 76f7fffb8d Clean up period identifiers.
SinglePeriodTimelines can all use the same identifier, as their identifier is
wrapped in a pair with the (fixed) source index when they are concatenated with
other sources' timelines.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130386297
2016-08-31 15:25:06 +01:00
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2016-08-31 15:25:06 +01:00