Change flag for PowerManagerClient

Replace the generic --enable-stub-interactive with a power manager
specific --power-stub flag.

This was separated out of an earlier CL that became network specific.

BUG=350170
R=derat@chromium.org, jennyz@chromium.org

Review URL: https://codereview.chromium.org/189623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256085 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256085