Add UTF-8 BOM for chrome_messages_json formatter.
Add UTF-8 BOM for chrome_messages_json formatter.

When uploading extension Chrome Web Store currently expects to find BOM
in UTF-8 files.

Review URL: https://codereview.chromium.org/11557029
camny125/grit-i18n,imuse2012/grit-i18n,imuse2012/grit-i18n,camny125/grit-i18n