Use the principal doing the load to decide on the URI to pass to content policies. This will help do the right thing for data:/javascript:/about:blank, as well as allow us to skip content policies altogether if the caller is system. Bug 388597, r+sr=sicking

docshell/base/nsDocShell.cpp

@@ -6380,9 +6380,11 @@ nsDocShell::InternalLoad(nsIURI * aURI,
     if (!context) {
         context =  mScriptGlobal;
     }
+    // XXXbz would be nice to know the loading principal here... but we don't
     rv = NS_CheckContentLoadPolicy(contentType,
                                    aURI,
                                    aReferrer,
+                                   nsnull,
                                    context,
                                    EmptyCString(), //mime guess
                                    nsnull,         //extra