Fixed crash in CloudPolicyManager.

The ComponentCloudPolicyService is created whenever a user signs in, but isn't
cleared when the user signs out; signing out and back in leads to this crash:

[2774:2774:1126/144024:FATAL:cloud_policy_manager.cc(111)] Check failed: !component_policy_service_.
[0x7f9aa98bba3c] policy::CloudPolicyManager::CreateComponentCloudPolicyService()
[0x7f9aa9518eaf] policy::UserCloudPolicyManager::Connect()
[0x7f9aa951a321] policy::UserPolicySigninService::InitializeUserCloudPolicyManager()
[0x7f9aa951a2af] policy::UserPolicySigninService::OnRefreshTokenAvailable()
[0x7f9aaab639d9] OAuth2TokenService::FireRefreshTokenAvailable()
[0x7f9aa91dbfc9] ProfileOAuth2TokenService::UpdateCredentials()
[0x7f9aa8c9647b] SigninManager::CompletePendingSignin()
[0x7f9aaa2d4b5f] OneClickSigninSyncStarter::ConfirmAndSignin()

BUG=None

Review URL: https://codereview.chromium.org/98433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238381 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238381