[inspector] Refactor inspector test

test/inspector/debugger/stepping-with-exposed-injected-script.js

 // Copyright 2017 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-inspector-scripts
 
 Protocol.Debugger.onPaused(message => {
-  let url = InspectorTest._scriptMap.get(message.params.callFrames[0].location.scriptId).url;
+  let url = InspectorTest.session._scriptMap.get(message.params.callFrames[0].location.scriptId).url;
   if (url !== 'test.js') {
     InspectorTest.log('InjectedSciptSource on stack.');
     InspectorTest.completeTest();
   }
   Protocol.Debugger.stepInto();
 });
 
 InspectorTest.setupScriptMap();
 Protocol.Debugger.enable();
 Protocol.Debugger.pause();
 Protocol.Runtime.evaluate({expression: 'console.log(42)//# sourceURL=test.js'})
   .then(() => InspectorTest.log('InjectedSciptSource was not reached'))
   .then(InspectorTest.completeTest);