chromeos: Add descriptions for power button histograms.

Add descriptions for the (newly-added)
Power.PowerButtonAcknowledgmentDelay and (old)
Power.PowerButtonDownTime metrics.

BUG=298983

Review URL: https://codereview.chromium.org/131493010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246068 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246068