This automatically searches the path for a source root (or takes one from the
command line), and then executes the current platform's GN binary inside that
source tree.

This will allow the user to have a "gn" command on their path, and allow our
scripts to run GN without having to bake-in the logic for finding the correct
platform's GN binary.

Review URL: https://codereview.chromium.org/66013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@233983 0039d316-1c4b-4281-b951-d872f2087c98