Bug 905364 - Don't call into AllowXULXBLForPrincipal during SafeJSContext initialization. r=bz

In the old world, we'd be saved by initializing the SafeJSContext early enough
that we'd short-circuit in the nsContentUtils::IsInitialized() check. That's not
the case anymore, so let's hande this explicitly.
Bug 905364 - Force the SafeJSContext to fire up in xpcshell. r=bz
Bug 905364 - Stop using the SafeJSContext in nsScriptSecurityManager::Init. r=mrbkap

With this patch, I've confirmed that we instantiate the SafeJSContext much later
in startup, during nsAppStartupNotifier::Observe (which ends up invoking an
XPCWrappedJS). As such, this should solve a number of our startup ordering woes.