Add chrome.debugger.getTargets method to discover available debug targets.

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

 // Copyright (c) 2013 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 debuggee;
 var protocolVersion = "1.0";
 
 chrome.test.runTests([
 
   function attach() {
     var extensionId = chrome.extension.getURL('').split('/')[2];
     debuggee = {extensionId: extensionId};
     chrome.debugger.attach(debuggee, protocolVersion, pass());
   },
 
   function attachToMissing() {
     var missingDebuggee = {extensionId: "foo"};
     chrome.debugger.attach(missingDebuggee, protocolVersion,
-        fail("No extension with given id " +
+        fail("No background page with given id " +
             missingDebuggee.extensionId + "."));
   },
 
   function attachAgain() {
     chrome.debugger.attach(debuggee, protocolVersion,
-        fail("Another debugger is already attached to the extension with id: " +
-            debuggee.extensionId + "."));
+        fail("Another debugger is already attached " +
+            "to the background page with id: " + debuggee.extensionId + "."));
   },
 
   function detach() {
     chrome.debugger.detach(debuggee, pass());
   },
 
   function detachAgain() {
     chrome.debugger.detach(debuggee,
-        fail("Debugger is not attached to the extension with id: " +
+        fail("Debugger is not attached to the background page with id: " +
             debuggee.extensionId + "."));
   }
 ]);