Revert of Add a mock class for DiskMountManagerObserver. disk_mount_manager_observer_unittests will…

Revert of Add a mock class for DiskMountManagerObserver. disk_mount_manager_observer_unittests will be rewrit… (patchset #17 id:320001 of https://codereview.chromium.org/2292473002/ )

Reason for revert:
memory leak detected in unittest

Original issue's description:
> Add a mock class for DiskMountManagerObserver.
> disk_mount_manager_unittest.cc will be rewritten using this class instead of gmock.
> 
> BUG=641943
> 
> Committed: https://crrev.com/7bec88d891d568be4392e3e5d74d04b79ff4e667
> Cr-Commit-Position: refs/heads/master@{#418182}

TBR=satorux@google.com,satorux@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=641943

[yamaguchi, 2016-09-13 10:30:46]

The CQ bit was checked by yamaguchi@chromium.org

[yamaguchi, 2016-09-13 10:30:47]

Created Revert of Add a mock class for DiskMountManagerObserver.
disk_mount_manager_observer_unittests will be rewrit…

[commit-bot: I haz the power, 2016-09-13 10:31:01]

The CQ bit was unchecked by commit-bot@chromium.org

[satorux1, 2016-09-13 11:29:51]

The CQ bit was checked by satorux@chromium.org

[commit-bot: I haz the power, 2016-09-13 11:30:03]

The CQ bit was unchecked by commit-bot@chromium.org

[yamaguchi, 2016-09-13 11:46:23]

Patch 2340433002 has already been applied for reverting the same change.
https://codereview.chromium.org/2340433002/
I will close this.