Change dcheck to an early return as a bad ipc message can reach this error condition.

      BUG=180909
TEST=N/A
Review URL: https://codereview.chromium.org/12758003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@187359 0039d316-1c4b-4281-b951-d872f2087c98