file_manager: Defer system suspend while file operations are in progress

Add 'power' permission to the manifest. (http://developer.chrome.com/extensions/power.html)
Use power API when the queue has jobs.

BUG=171949

Review URL: https://codereview.chromium.org/110193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239785