- Pull in a trimmed down version of vs_toolchain.py from Chromium.

- Drop in toolchain_vs2013.hash from Chromium to use Chromium's
  current toolchain.

- Add a very hacky vs_env.py to pull in Visual Studio. This is
  loosely based off a handful of lines of Chromium's
  tools/clang/scripts/update.py. This (and vs_toolchain.py) depends
  on gyp which is now pulled in via DEPS.

BUG=430237

Change-Id: Ic29cbb15e19a99616cfe778d0778b9a71c45338a
Reviewed-on: https://boringssl-review.googlesource.com/3900
Reviewed-by: Adam Langley <agl@google.com>