Bug 774607 - Remove use of JS_{Is,Make}SystemObject from Gecko. r=bz

We currently set this for system globals and anything whose parent
chain leads to a system global. Maybe this was relevant before, but
with CPG this is just equivalent to asking whether the object is in
a system compartment. And the only place where we _check_ this bit
is immediately after checking for a system compartment, in
WrapperFactory. So AFAICT this can go away entirely.

ipc/testshell/XPCShellEnvironment.cpp

@@ -1088,9 +1088,7 @@ XPCShellEnvironment::Init()
 
     nsCOMPtr<nsIXPConnectJSObjectHolder> holder;
     rv = xpc->InitClassesWithNewWrappedGlobal(cx, backstagePass,
-                                              principal,
-                                              nsIXPConnect::
-                                                  FLAG_SYSTEM_GLOBAL_OBJECT,
+                                              principal, 0,
                                               getter_AddRefs(holder));
     if (NS_FAILED(rv)) {
         NS_ERROR("InitClassesWithNewWrappedGlobal failed!");