shiny/widget/flex: justify-content support

Switch to rounding pixels to the closest integer to match Chrome's
output.

Change-Id: I94fea521e3ac40ba642bf5013f3cd48d477581b6
Reviewed-on: https://go-review.googlesource.com/23018
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed