Depend on content/test for unit test setup/teardown

This will allow us to organize code more logically on the chromium side.
In Chromium, content is the embedder of Blink and provides the platform
implementation.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/18711009

git-svn-id: svn://svn.chromium.org/blink/trunk@154586 bbb929c8-8fbe-4397-9dbb-9b2b20218538