Mojo: rename limits.h to constants.h.

(As suggested by darin in a code review.)

R=sky@chromium.org

Review URL: https://codereview.chromium.org/103593004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241022 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241022