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Refactor disk_mount_manager_unittest by replacing gmock with a dedicated mock...

Refactor disk_mount_manager_unittest by replacing gmock with a dedicated mock class of DiskMountManagerObserver.

This change is based on
https://codereview.chromium.org/2292473002/
which was once merged but reverted by
https://codereview.chromium.org/2340433002/

BUG=641943

Review-Url: https://codereview.chromium.org/2333983004
Cr-Commit-Position: refs/heads/master@{#418478}