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This will eliminate the observer needing to make an immediate
async call back to the MediaTransferProtocolManager to obtain
the MtpStorageInfo object from its identifier.

This is a preparation work for MTP servicification.

BUG=769630

Change-Id: I239ed96d42c4e496f97d311a689704e34c55edda
Reviewed-on: https://chromium-review.googlesource.com/882494
Commit-Queue: Donna Wu <donna.wu@intel.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533232}