Port 'external startup data' flag from gyp to gn.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/316363004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21721 ce2b1a6d-e550-0410-aec6-3dcde31c8c00