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Since the instance of CrxComponent is provided by the clients of the
update_client::UpdateClient, it is possible that instance is missing in
some cases, such as when a CRX is uninstalled. That leads to corner
cases such as calls to UpdateClient::GetCrxUpdateState returning an
error, because an instance of CrxUpdateItem could not be returned.

This is a mechanical change to replace the std::unique_ptr wrapper of
the CrxComponent with a base::Optional wrapper, so that CrxUpdateItem
could have correct value semantics.

BUG=869663

Change-Id: I6aa805c2784af5ed959c8777f724322763404cb0
Reviewed-on: https://chromium-review.googlesource.com/1173018
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Minh Nguyen <mxnguyen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583913}