GN: Autodetect Visual Studio versions

This searches the local system for Visual Studio versions like GYP. It enables specifically requesting one version, as well as overriding the directory like GYP.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/126073005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243612 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243612