Add SpellCheck.SpellingService.Enabled histogram.

Code to record stats for the SpellCheck.SpellingService.Enabled histogram
was added in https://chromiumcodereview.appspot.com/12335019/. This CL adds
the histogram to the main list of histograms so that other systems will
pick it up.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/25752007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226913 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226913
Build cast_unittest

Build cast targets on all bots.

R=justinlin@chromium.org

Review URL: https://codereview.chromium.org/25891004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226914 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226914