Add CPU usage information to the chrome.system.cpu extension API.

BUG=339885
TESTED=Unit test updated, manual tests at extension console.

Review URL: https://codereview.chromium.org/136123009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249913 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249913