Bug 1148593 - Create async stack in callback objects. r=bz, r=fitzgen

browser/devtools/canvasdebugger/test/browser_canvas-frontend-call-stack-01.js

@@ -28,8 +28,10 @@ function* ifTestingSupported() {
 
   isnot($(".call-item-stack", callItem.target), null,
     "There should be a stack container available now for the draw call.");
-  is($all(".call-item-stack-fn", callItem.target).length, 4,
-    "There should be 4 functions on the stack for the draw call.");
+  // We may have more than 4 functions, depending on whether async
+  // stacks are available.
+  ok($all(".call-item-stack-fn", callItem.target).length >= 4,
+     "There should be at least 4 functions on the stack for the draw call.");
 
   ok($all(".call-item-stack-fn-name", callItem.target)[0].getAttribute("value")
     .includes("C()"),