Mojo: Fix some incorrect DCHECKS

BUG=None
R=ben@chromium.org

Review URL: https://codereview.chromium.org/107873003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242273 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242273