Bug 856497 - Fix dynamic viewport sizing for margin changes. r=kats

The viewport wasn't being recalculated when the viewport margins changed, and
was also subject to some rounding errors. Round off the values before
comparing them (as they're screen pixels), and make sure to update the viewport
when the margins change. Margins were also not correctly being altered when
in overscroll, which could cause bottom-aligned fixed position content to be
incorrectly offset.