Mark all GN targets as external.

This prevents GN from writing any GYP files in gyp mode in the current build so I can land the build integration separately from the target generation.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/73303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235256 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235256