Mojo: Add a C++ DataPipe wrapper paralleling the MessagePipe wrapper.

I should also write "end-to-end" data pipe tests ... but that will be a
nontrivial project.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/130633005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245647 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245647