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As a preparation of VolumeManager refactoring, this CL allows injecting
DiskMountManager to MounteddiskMonitor. Along with the change, this CL also
adds unittest for it.

BUG=279276
TEST=Ran unit_tests

Review URL: https://chromiumcodereview.appspot.com/23676008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222072 0039d316-1c4b-4281-b951-d872f2087c98