static: detect platform to hide/show unix/windows instructions

Currently both "testUnix" and "testWindows" elements are shown simultaneously.
This change detects the correct platform to hide/show the appropriate
elements based on the viewer's browser.

Change-Id: I3afee7507d61b2d756127166caede85adcd531b8
Reviewed-on: https://go-review.googlesource.com/35103
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed