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

chrome/browser/extensions/api/debugger/debugger_api_constants.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/extensions/api/debugger/debugger_api_constants.h"
 
 namespace debugger_api_constants {
 
 const char kOnEvent[] = "debugger.onEvent";
 const char kOnDetach[] = "debugger.onDetach";
 
 const char kAlreadyAttachedError[] =
-    "Another debugger is already attached to the tab with id: *.";
+    "Another debugger is already attached to the * with id: *.";
 const char kAttachToWebUIError[] =
     "Can not attach to the page with the \"*://\" scheme.";
-const char kNoTabError[] = "No tab with given id *.";
+const char kNoTargetError[] = "No * with given id *.";
+const char kInvalidTargetError[] =
+    "Either tab id or extension id must be specified.";
 const char kNotAttachedError[] =
-    "Debugger is not attached to the tab with id: *.";
+    "Debugger is not attached to the * with id: *.";
 const char kProtocolVersionNotSupportedError[] =
     "Requested protocol version is not supported: *.";
+const char kSilentDebuggingRequired[] =
+    "Cannot attach to an extension, --* command line option required.";
+
+const char kTabTargetType[] = "tab";
+const char kExtensionTargetType[] = "extension";
 
 }  // namespace debugger_api_constants