Don't use calling context if it is not WebKit's

DRQS 41162567

The calling context used for activeContext() might be a non-WebKit context, in
which case we treat it like it doesn't exist at all, and we fall back to the
entered context.
Don't assert domWindow in FeatureObserver

DRQS 41162567

FeatureObserver is not very important to us.  It is ok if this is a no-op when
the domWindow is null.