Bug 1070696 - Rename xpc::GetNativeForGlobal to xpc::NativeGlobal and make it accept non-global JSObjects. r=smaug

The method is infallible, and making the callers invoke
js::GetGlobalForObjectCrossCompartment manually is dumb.
Bug 1070696 - Implement a JSObject* override to AutoJSAPI::Init(). r=smaug