Improve Windows build instructions

This change removes a stray reference to VS 2015, clarifies when VS
2017 became required, and clarifies which VS 2017 components must be
installed.

Bug: 699375
Change-Id: Ia06240a4581afb784a25330170c61ce3f124cf17
Reviewed-on: https://chromium-review.googlesource.com/724284
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509631}
1 file changed