Move root GN build file, add files for the GN tutorial

This moves the root GN build file to the source root so it's easier to find.

The files are to support the tutorial at https://code.google.com/p/chromium/wiki/GNQuickStart

R=jam@chromium.org

Review URL: https://codereview.chromium.org/120923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242555 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242555