GN: Depend on actual v8, not a stub target

V8 has GN build files now, we don't need this stub any more.

Review URL: https://codereview.chromium.org/329813002

git-svn-id: svn://svn.chromium.org/blink/trunk@176045 bbb929c8-8fbe-4397-9dbb-9b2b20218538