Add an assert that DisplayItem types are kept in sync with PaintPhase

This patch adds an assert that we have kept DisplayItem's Type up to
date with the PaintPhase enum. This is in addition to comments in both
DisplayItem and PaintPhase.

Review URL: https://codereview.chromium.org/697473002

git-svn-id: svn://svn.chromium.org/blink/trunk@184691 bbb929c8-8fbe-4397-9dbb-9b2b20218538