Mojo: Fix bug in closing MessagePipes.

(Also add a test to exercise that code path ... though it doesn't fail even with
the old, incorrect code.)

R=darin@chromium.org, darin

Review URL: https://codereview.chromium.org/40923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232748 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232748