Load the Chrome OS TTS engine manifest from the file system isntead of resources.

chrome/common/extensions/extension_constants.h

 // 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.
 
 #ifndef CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_
 #define CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_
 
 #include <string>
 
 #include "base/basictypes.h"
+#include "base/files/file_path.h"
 #include "url/gurl.h"
 
 namespace extension_urls {
 
 // This returns the compile-time constant webstore update url specific to
 // Chrome. Usually you should prefer using GetWebstoreUpdateUrl.
 GURL GetDefaultWebstoreUpdateUrl();
 
 // Field to use with webstore URL for tracking launch source.
 extern const char kWebstoreSourceField[];
@@ skipping 164 matching lines @@
   APP_LAUNCH_BUCKET_INVALID
 };
 
 // The extension id of the ChromeVox extension.
 extern const char kChromeVoxExtensionId[];
 
 #if defined(OS_CHROMEOS)
 // Path to preinstalled ChromeVox screen reader extension (relative to
 // |chrome::DIR_RESOURCES|).
 extern const char kChromeVoxExtensionPath[];
-// Name of ChromeVox manifest file.
-extern const char kChromeVoxManifestFilename[];
-// Name of ChromeVox guest manifest file.
-extern const char kChromeVoxGuestManifestFilename[];
+// Name of the manifest file in an extension when a special manifest is used
+// for guest mode.
+extern const base::FilePath::CharType kGuestManifestFilename[];

[not at google - send to devlin, 2015/06/01 17:39:37]

FilePath::CharType is more correct I suppose, but I'd prefer to keep it the same
as the rest of the file (e.g. note kChromeVoxExtensionPath,
kSpeechSynthesisExtensionPath, etc) as a const char[].

 // Extension id, path (relative to |chrome::DIR_RESOURCES|) and IME engine
 // id for the builtin-in Braille IME extension.
 extern const char kBrailleImeExtensionId[];
 extern const char kBrailleImeExtensionPath[];
 extern const char kBrailleImeEngineId[];
 // Path to preinstalled Connectivity Diagnostics extension.
 extern const char kConnectivityDiagnosticsPath[];
 extern const char kConnectivityDiagnosticsLauncherPath[];
 // The extension id of the first run dialog application.
 extern const char kFirstRunDialogId[];
@@ skipping 32 matching lines @@
 // The path part of the file system url used for media file systems.
 extern const char kMediaFileSystemPathPart[];
 
 // The key used for signing some pieces of data from the webstore.
 extern const uint8 kWebstoreSignaturesPublicKey[];
 extern const int kWebstoreSignaturesPublicKeySize;
 
 }  // namespace extension_misc
 
 #endif  // CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_