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Some recent reports indicate that the Sync server might be sending
duplicate client tag hashes. This is considered "invalid data", and is
so far not handled well by the client. These metrics will help us
evaluate how common this case is, and also whether the duplicates are
within a single GetUpdates response, or across multiple responses
(within a full paginated GetUpdates cycle).

Bug: 872360
Change-Id: Id24baa351c80f2eb1ab59f890d50d6603cb89e6b
Reviewed-on: https://chromium-review.googlesource.com/1203935Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590328}