Bug 801120 - Support T* WeakPtr<T>::get(). r=Ms2ger, r=ehsan, a=tef+
Bug 821804 - Allow unitialized WeakPtrs to work properly. r=Waldo, a=tef+

This ensures that the WeakPtr always points at a valid object and avoids the
need to test for that during operations.
Bug 826410 - Add a missing interface to MediaElement's QI impl. r=jst, a=tef+
Bug 859307 - [updates] wrong behavior when the manifest changes but not the packaged zip. r=ferjm, a=tef+