Classes used outside their components need to be exported.

This fixes the Mac 64 component build after r161693.

BUG=297477

Review URL: https://codereview.chromium.org/68223002

git-svn-id: svn://svn.chromium.org/blink/trunk@161701 bbb929c8-8fbe-4397-9dbb-9b2b20218538