Run GN as part of runhooks.

This runs GN in GYP mode before actually running GYP.

Currently, GN does not produce any files used by the build, these will be added in a second step.

BUG=
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/62803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236542 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236542