Mojo: Add a Mutex class in public/utility.

(Reland. Originally committed r250513. Reverted r250513. Fix issues that only arose with gcc.)

R=davemoore@chromium.org
TBR=davemoore@chromium.org

Review URL: https://codereview.chromium.org/160053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250628 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250628