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

chrome/common/extensions/extension_constants.h

Index: chrome/common/extensions/extension_constants.h
diff --git a/chrome/common/extensions/extension_constants.h b/chrome/common/extensions/extension_constants.h
index 42d3d17505829216098b070f5f5e7a545e499c26..cf65b72b3c8c637f87f937d10ab9f7ec57336e70 100644
--- a/chrome/common/extensions/extension_constants.h
+++ b/chrome/common/extensions/extension_constants.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/basictypes.h"
+#include "base/files/file_path.h"
 #include "url/gurl.h"
 
 namespace extension_urls {
@@ -192,10 +193,9 @@ extern const char kChromeVoxExtensionId[];
 // 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[];