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This will make it easier to use this factory when testing code that uses
SetCanonicalCookie() and other methods that take unique_ptrs to
CanonicalCookies.

The factory method uses base::WrapUnique to create the unique_ptr so
that the constructor can be made private. I plan on updating the other
factory methods to use base::WrapUnique instead of make_unique in a
follow-up CL.

Bug: 1102874
Change-Id: I3e84fbc68ce95438a5adfdac46573df9efe4e38d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559311Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Cr-Commit-Position: refs/heads/master@{#831072}