If download is not possible, don't show scene2 on /new.

See bugs 891562 and 919788.

This also prevents scene2 from appearing if you manually update the location hash. Both of these scenarios could cause bogus download stats when the page is loaded in firefox-latest or fxos or an unsupported platform.
Merge pull request #1267 from gauthierm/fix-bad-download-events

If using firefox-latest or an unsupported platform, don't show scene2 on page load.