DevTools: provide the debugger detach reason in chrome.debugger.onDetach extension API.

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 51 matching lines @@
         fail("Debugger is not attached to the tab with id: " + tabId + "."));
   },
 
   function detachAgain() {
     chrome.debugger.detach(debuggee,
         fail("Debugger is not attached to the tab with id: " + tabId + "."));
   },
 
   function closeTab() {
     chrome.tabs.create({url:"inspected.html"}, function(tab) {
-      function onDetach(debuggee) {
+      function onDetach(debuggee, reason) {
         chrome.test.assertEq(tab.id, debuggee.tabId);
+        chrome.test.assertEq("target_closed", reason);
         chrome.debugger.onDetach.removeListener(onDetach);
         chrome.test.succeed();
       }
 
       var debuggee2 = {tabId: tab.id};
       chrome.debugger.attach(debuggee2, protocolVersion, function() {
         chrome.debugger.onDetach.addListener(onDetach);
         chrome.tabs.remove(tab.id);
       });
     });
   },
 
   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);
     });
   }
 
 ]);