Rename this gyp variable to something meaningful to better explain what
it actually does.

BUG=184431

Review URL: https://chromiumcodereview.appspot.com/12605007
Patch from Richard Coles <torne@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13916 ce2b1a6d-e550-0410-aec6-3dcde31c8c00