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";
 
@@ skipping 19 matching lines @@
   },
 
   function detach() {
     chrome.debugger.detach(debuggee, pass());
   },
 
   function detachAgain() {
     chrome.debugger.detach(debuggee,
         fail("Debugger is not attached to the extension with id: " +
             debuggee.extensionId + "."));
+  },
+
+  function discoverOwnExtension() {
+    chrome.debugger.getTargets(function(targets) {
+      var target = targets.filter(
+          function(t) {
+            return t.type == 'extension' && t.title == 'Extension Debugger';
+          })[0];
+      if (target) {
+        chrome.debugger.attach({targetId: target.id}, protocolVersion, pass());
+      } else {
+        chrome.test.fail("No extension found");
+      }
+    });
   }
 ]);