[debug] Extend a test and fix some typos.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index cb1f34f390e50b54eab226359e504ec39ca10c7b..4a92dfabdb62f89403f4daea7b8ef5d95d8a1502 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -1945,7 +1945,7 @@ void Debug::CallEventCallback(v8::DebugEvent event,
     MaybeHandle<Object> result =
         Execution::Call(isolate_, Handle<JSFunction>::cast(event_listener_),
                         global, arraysize(argv), argv);
-    CHECK(!result.is_null());  // Listeners may not throw.
+    CHECK(!result.is_null());  // Listeners must not throw.
   }
   in_debug_event_listener_ = previous;
 }