Mojo: (POSIX) Pass channel handle to child.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/103113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238487