Improve reliability of detecting small devices when loading Differential changesets

Summary: 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.

Test Plan:
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).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9969, T10229

Differential Revision: https://secure.phabricator.com/D15135