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We are planning to use ephemeral profiles to implement this method
in the future, but their creation is async. OTR profiles which are
currently used have synchronous creation, which is why this method
was synchronous so far.

This CL updates the implementation to be async, but does not yet
make any other changes. In later CLs we will plumb through the
callback to the async ephemeral profile creation.

Doc for context:
https://docs.google.com/document/d/1fnTD_smbp5vzWTLAuXagVsqm_C5607DPbKcfx6KbCTY

Bug: 1086935
Change-Id: I19c72bd002574ba8e6c810f3fff25d974e1f852e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230461Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775106}