[Extensions] Validate file existence for chrome url overrides

extensions/common/manifest_constants.h

 // 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.
 
 #ifndef EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_
 #define EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_
 
 #include "build/build_config.h"
 
 namespace extensions {
@@ skipping 249 matching lines @@
 extern const char kCannotChangeExtensionID[];
 extern const char kCannotClaimAllHostsInExtent[];
 extern const char kCannotClaimAllURLsInExtent[];
 extern const char kCannotScriptGallery[];
 extern const char kCannotScriptSigninPage[];
 extern const char kCannotUninstallManagedExtension[];
 extern const char kChromeVersionTooLow[];
 extern const char kDevToolsExperimental[];
 extern const char kDisabledByPolicy[];
 extern const char kExpectString[];
+extern const char kFileNotFound[];
 extern const char kInvalidAboutPage[];
 extern const char kInvalidAboutPageExpectRelativePath[];
 extern const char kInvalidAllFrames[];
 extern const char kInvalidAppIconColor[];
 extern const char kInvalidBackground[];
 extern const char kInvalidBackgroundAllowJsAccess[];
 extern const char kInvalidBackgroundCombination[];
 extern const char kInvalidBackgroundScript[];
 extern const char kInvalidBackgroundScripts[];
 extern const char kInvalidBackgroundInHostedApp[];
@@ skipping 211 matching lines @@
 extern const char kIllegalPlugins[];
 extern const char kInvalidFileSystemProviderMissingCapabilities[];
 extern const char kInvalidFileSystemProviderMissingPermission[];
 #endif
 
 }  // namespace manifest_errors
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_