Check for possibility of NULL WebUI returns in PrintPreviewDialogController.

This is a case that is easily hit with the IPC fuzzer, and is easy to
handle. Fixing this helps the testcases to run further.

BUG=337535

Review URL: https://codereview.chromium.org/131343005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246948 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246948