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 {

tests/html/panic.html

@@ -0,0 +1,16 @@
+<!-- To exercise these tests, set the `dom.testbinding.enabled` to true.
+     It is expected that the browser will not abort due to failed JS engine assertions. -->
+
+<!-- Straightforward test - invoking a panic from a toplevel script execution -->
+<script>
+(new TestBinding()).panic();
+</script>
+
+<!-- invoking a panic from an event handler which is invoked by native code -->
+<!--<iframe src="data:,hi there" onload="(new TestBinding()).panic()"></iframe>-->
+
+<!-- invoking a panic from an event handler which is invoked by script -->
+<!--<div onclick="(new TestBinding()).panic()"></div>
+<script>
+document.querySelector('div').click();
+</script>-->