Gn: Add support to generate Visual Studio 2017 solutions.

- Add a new command line vs2017 to generate 2017 solutions
- Keep 2015 as the default.
- Update the toolset version.

BUG=683729

Review-Url: https://codereview.chromium.org/2738823002
Cr-Original-Commit-Position: refs/heads/master@{#456443}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c20ee868a144b9f01eaeae39bba8e9921dee2635
3 files changed
tree: da700866f393a0402dd40192f90e437bff081b7f
  1. tools/