Allow chrome.debugger API to attach to extension background pages.

chrome/test/data/extensions/api_test/debugger/background.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var pass = chrome.test.callbackPass;
 var fail = chrome.test.callbackFail;
 
 var tabId;
 var debuggee;
 var protocolVersion = "1.0";
@@ skipping 80 matching lines @@
     });
   },
 
   function attachToWebUI() {
     chrome.tabs.create({url:"chrome://version"}, function(tab) {
       var debuggee = {tabId: tab.id};
       chrome.debugger.attach(debuggee, protocolVersion,
           fail("Can not attach to the page with the \"chrome://\" scheme."));
       chrome.tabs.remove(tab.id);
     });
-  }
+  },
 
+  function attachToMissing() {
+    var missingDebuggee = {tabId: -1};
+    chrome.debugger.attach(missingDebuggee, protocolVersion,
+        fail("No tab with given id " + missingDebuggee.tabId + "."));
+  },
+
+  function attachToExtensionWithNoSilentFlag() {
+    debuggeeExtension = {extensionId: "foo"};
+    chrome.debugger.attach(debuggeeExtension, protocolVersion,
+        fail("Cannot attach to an extension, " +
+            "--extension-debugging-silent command line option required."));
+  },
 ]);