Ref T10229. Fixes T9969. We make a decision about 1up vs 2up pretty early, and sometimes the viewport size reads as larger than the device, so we incorrectly select 2up when the proper setting would be 1up.

I can reproduce this by, e.g., reloading a lot on an iPhone 4s in iOS Simulator running iOS 9.2. Sometimes it picks 2-up. I added logging to show that the viewport dimension read was the issue.

After this change, it always selects 1-up (window.screen.availWidth is defined and sensible on the device).