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This is a reland of 956715c7

Original change's description:
> Fix sync protocol violation about birthday requirements
> 
> 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/+/1599456
> Reviewed-by: Marc Treib <treib@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#658540}

Bug: 923285
Change-Id: I38101a4e534d80a6cf4d29dbdaeac078e4c03212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607806Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659025}