[debugger] Add test for Debug.scripts without listener.

This adds a test case that ensures calling Debug.scripts without any
listener attached fails gracefully. For now we are throwing the string
"illegal access", this might change in the future to be a dedicated
exception.

R=yangguo@chromium.org
TEST=mjsunit/debug-scripts-throw

Review URL: https://codereview.chromium.org/1411193002

Cr-Commit-Position: refs/heads/master@{#31377}

test/mjsunit/debug-scripts-throw.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug
+
+// Get the Debug object exposed from the debug context global object.
+Debug = debug.Debug;
+
+assertThrows("Debug.scripts()");
+Debug.setListener(function(){});
+
+assertDoesNotThrow("Debug.scripts()");
+Debug.setListener(null);