Bug 826325 - preventDefault all mozbrowserwindowopen events so that Gecko won't attempt to open them

With the patch in Bug 826325, Gecko is altered so that if you don't call preventDefault on these events, they'll fall through and be sent to Gecko's normal window.open code code paths:

http://mxr.mozilla.org/mozilla-central/source/xpfe/appshell/src/nsContentTreeOwner.cpp#845

To maintain its old behavior, Gaia should just always call preventDefault()