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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/2333983004/
which was once merged but reverted by
https://codereview.chromium.org/2343593002/

BUG=641943
TEST=chromeos_unittest build with is_asan and is_lsan = true in gn args, and run with ASAN_OPTIONS="detect_leaks=1 symbolize=1" environment variable.

Review-Url: https://codereview.chromium.org/2341923003
Cr-Commit-Position: refs/heads/master@{#418811}