GN support for generating VS2013 project files

Optional Visual Studio version can now be specified for "gn gen".
The default remains Visual Studio 2015.

BUG=

Review URL: https://codereview.chromium.org/1713363002

Cr-Original-Commit-Position: refs/heads/master@{#376849}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d0bc0fa329d753da4fb333c572361f575411b8a7
4 files changed
tree: a35d80ba7a5c851fbeabc388322d97a5318afaad
  1. tools/