policy: Fix crash in UserCloudPolicyManagerFactory::RegisterForOffTheRecordProfile.

After the OffTheRecordProfile was destructed, an invalid stale pointer to the profile was still in the UserCloudPolicyManagerFactory::managers_.

When a new OffTheRecordProfile was created with coincidentally the same address, then the DCHECK in Register triggered.

Now, during shutdown of the OffTheRecordProfile the respective entry is removed from managers_.

BUG=315266
R=dconnelly@chromium.org

Review URL: https://codereview.chromium.org/59883007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233615 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233615