Fix naming of one accelerated compositing stat used in benchmarking

      The name was slightly misleading; instead of counting the total number of
cc/layers in the layer tree, it actually counts the number of layers that are
issued to be drawn.


Review URL: https://chromiumcodereview.appspot.com/11418245

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@170368 0039d316-1c4b-4281-b951-d872f2087c98