Implemented the rest of loading/parsing logic for extension i18n:...

chrome/browser/extensions/extension_l10n_util.h

Index: chrome/browser/extensions/extension_l10n_util.h
===================================================================
--- chrome/browser/extensions/extension_l10n_util.h	(revision 24127)
+++ chrome/browser/extensions/extension_l10n_util.h	(working copy)
@@ -41,6 +41,16 @@
                      Extension* extension,
                      std::string* error);
 
+// Loads messages file for default locale, and application locale (application
+// locale doesn't have to exist).
+// It creates simplified in-memory representation of name-value pairs, where
+// value part is actual message with placeholders resolved.
+// Returns true if it can load default locale messages file, and all messages
+// are valid.
+bool LoadMessageCatalogs(const FilePath& locale_path,
+                         Extension* extension,
+                         std::string* error);
+
 }  // namespace extension_l10n_util
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_L10N_UTIL_H_