GN: Add Windows support to bootstrap.py

Previously GN could only be bootstrapped on Linux/Mac.
This CL adds support for Windows (Visual Studio only at this time).

Note that bootstrap.py expects to be run inside a VS2015 Command Prompt
(i.e. vcvarsall.bat has been run)

Example:
> call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\vc\vcvarsall.bat" x86
> tools/gn/bootstrap/bootstrap.py

BUG=

Review-Url: https://codereview.chromium.org/2064513002
Cr-Original-Commit-Position: refs/heads/master@{#403499}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3a32f0297bbd2c5673cf02923244ecfeda927b32
4 files changed
tree: 923d04df71876dc27fef0cf6628365a21d3fe13f
  1. tools/