<picture>: Update the <img> element when a <source> changes

R=abarth@chromium.org
CC=yoav@yoav.ws
BUG=233751

Review URL: https://codereview.chromium.org/290203002

git-svn-id: svn://svn.chromium.org/blink/trunk@174731 bbb929c8-8fbe-4397-9dbb-9b2b20218538