Convert relative "poster" attribute to absolute for HTML5 video.
Convert relative "poster" attribute to absolute for HTML5 video.

This attribute was missed previously and is the pre-roll image for
<video> tags. Also, only make attributes absolute that were previously
specified.

BUG=376107
R=kuan@chromium.org

Review URL: https://codereview.chromium.org/391843003

dalmirdasilva/dom-distiller,dalmirdasilva/dom-distiller,dalmirdasilva/dom-distiller,dalmirdasilva/dom-distiller,dalmirdasilva/dom-distiller