Add a manual test for panicking while JS stack frames exist.

components/script/dom/testbinding.rs

@@ -580,6 +580,8 @@ impl TestBindingMethods for TestBinding {
             ptr::write_volatile(p, 0xbaadc0de);
         }
     }
+
+    fn Panic(&self) { panic!("explicit panic from script") }
 }
 
 impl TestBinding {

components/script/dom/webidls/TestBinding.webidl

@@ -455,6 +455,8 @@ interface TestBinding {
   static void funcControlledStaticMethodEnabled();
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
+
+  void panic();
 };
 
 partial interface TestBinding {