Improve Windows GN build setup.

Properly setup a new output build directory on Windows (previously you had to manually copy some files into it). It now creates the environment block and the gyp-win-tool.

The include and library paths are moved to the command line and removed from the environment which is cleaner and more explicit.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/34603009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230223 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230223