Adds timelineModified event and CSSRenderer.prototype.prerender.

Squashed commit of the following:

commit 880c9122d2a2ce58cb3dacfd103089e23e4088a1
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Jul 25 20:52:50 2013 -0700

    Adds null check in invalidatePropertyCache.

commit 3e0e6ad48cada7f100cdbd53288741bff8705e13
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Jul 25 20:49:41 2013 -0700

    Adds CSSRenderer#prerender.

commit 3cccdc51d889d534b640b51b94d33dfc04f95cce
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Jul 25 20:20:01 2013 -0700

    Adds timelineModified event.