Bug 1009111 - Enable vertical homescreen by default

https://bugzilla.mozilla.org/show_bug.cgi?id=1009111

On try: https://tbpl.mozilla.org/?tree=Try&rev=4108119e9bae
On try again: https://tbpl.mozilla.org/?tree=Try&rev=5d11434b3718
And again: https://tbpl.mozilla.org/?tree=Try&rev=9899570a1992