Add the about_page key to shared_module manifest

extensions/common/manifest_constants.cc

 // Copyright 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.
 
 #include "extensions/common/manifest_constants.h"
 
 namespace extensions {
 
 namespace manifest_keys {
 
+const char kAboutPage[] = "about_page";
 const char kAllFrames[] = "all_frames";
 const char kAltKey[] = "altKey";
 const char kApp[] = "app";
 const char kAutomation[] = "automation";
 const char kBackgroundAllowJsAccess[] = "background.allow_js_access";
 const char kBackgroundPage[] = "background.page";
 const char kBackgroundPageLegacy[] = "background_page";
 const char kBackgroundPersistent[] = "background.persistent";
 const char kBackgroundScripts[] = "background.scripts";
 const char kBluetooth[] = "bluetooth";
@@ skipping 241 matching lines @@
     "The sign-in page cannot be scripted.";
 const char kChromeVersionTooLow[] =
     "This extension requires * version * or greater.";
 const char kDisabledByPolicy[] =
     "This extension has been disabled by your administrator.";
 const char kExpectString[] = "Expect string value.";
 const char kExperimentalFlagRequired[] =
     "Loading extensions with 'experimental' permission is turned off by "
     "default. You can enable 'Experimental Extension APIs' "
     "by visiting chrome://flags.";
+const char kInvalidAboutPage[] = "Invalid value for 'about_page'.";
+const char kInvalidAboutPageExpectRelativePath[] =
+    "Invalid value for 'about_page'. Value must be a relative path.";
 const char kInvalidAllFrames[] =
     "Invalid value for 'content_scripts[*].all_frames'.";
 const char kInvalidBackground[] =
     "Invalid value for 'background_page'.";
 const char kInvalidBackgroundAllowJsAccess[] =
     "Invalid value for 'background.allow_js_access'.";
 const char kInvalidBackgroundCombination[] =
     "The background.page and background.scripts properties cannot be used at "
     "the same time.";
 const char kInvalidBackgroundScript[] =
@@ skipping 442 matching lines @@
 const char kWebRequestConflictsWithLazyBackground[] =
     "The 'webRequest' API cannot be used with event pages.";
 #if defined(OS_CHROMEOS)
 const char kIllegalPlugins[] =
     "Extensions cannot install plugins on Chrome OS";
 #endif
 
 }  // namespace manifest_errors
 
 }  // namespace extensions