Revert 236542 "Run GN as part of runhooks."

> 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

TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/79023008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236543 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236543