We should create the Prerenderer lazily

There's no reason to create this object eagerly on Document creation.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/25091004

git-svn-id: svn://svn.chromium.org/blink/trunk@158468 bbb929c8-8fbe-4397-9dbb-9b2b20218538