Require external V8 snapshots to be inside the webkit_unit_tests apk

V8 gets initialized when webkit_unit_tests is run, which requires V8's external snapshots to be inside the test's apk.

BUG=421063

Review URL: https://codereview.chromium.org/706453002

git-svn-id: svn://svn.chromium.org/blink/trunk@184934 bbb929c8-8fbe-4397-9dbb-9b2b20218538