chromeos: Avoid PowerPolicyController segfault.

Destroy PowerPolicyController before PowerManagerClient to
avoid a crash triggered by r252002.

BUG=345457
R=satorux@chromium.org

Review URL: https://codereview.chromium.org/174623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252683