UMA: Add labels to new items of Blink UseCounter.

http://src.chromium.org/viewvc/blink/trunk/Source/core/page/UseCounter.h?view=diff&r1=160513&r2=158212&diff_format=h

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/47803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231297 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231297