Bug 13855121 - Do not race with gcBytes, r=jonco

--HG--
extra : rebase_source : 32c6b04e6369092c4de7ff48e72e9bf29fcdf947

js/src/jit-test/tests/debug/bug1109328.js

@@ -4,4 +4,5 @@ try {
 g = newGlobal()
 g.parent = this
 g.eval("Debugger(parent).onExceptionUnwind=(function(){})");
-gcparam("maxBytes", gcparam("gcBytes"));
+gcparam("maxBytes", gcparam("maxBytes") - 8);
+gc();