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The sync (store) birthday can only be empty during the very initial
interactions with the server, when control datatypes are downloaded,
after enabling sync (or the user signing in, for sync-the-transport
mode).

If, during startup, the birthday is empty, we should throw away all
sync metadata and redownload everything. One way to do so is to
regenerate the cache GUID, which datatypes detect as mismatch and
leads to all sync metadata being cleared.

As can seen in newly added tests, the scenario seems hard to
reproduce, but we have server-side evidence that some users run into
this issue (and would even DCHECK-fail for DCHECK-enabled builds).

Bug: 923285
Change-Id: If9e4807a086001931fd65f3e12a1ab53c4a33c9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599456Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658540}