DevTools: Remove console last result $_ on console clear.

Source/core/inspector/InjectedScriptSource.js

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 341 matching lines @@
     _parseObjectId: function(objectId)
     {
         return nullifyObjectProto(InjectedScriptHost.eval("(" + objectId + ")"));
     },
 
     /**
      * @param {string} objectGroupName
      */
     releaseObjectGroup: function(objectGroupName)
     {
+        if (objectGroupName === "console")
+            delete this._lastResult;
         var group = this._objectGroups[objectGroupName];
         if (!group)
             return;
         for (var i = 0; i < group.length; i++)
             this._releaseObject(group[i]);
         delete this._objectGroups[objectGroupName];
     },
 
     /**
      * @param {string} methodName
@@ skipping 1327 matching lines @@
      */
     _logEvent: function(event)
     {
         inspectedWindow.console.log(event.type, event);
     }
 }
 
 injectedScript._commandLineAPIImpl = new CommandLineAPIImpl();
 return injectedScript;
 })