Add the picture element

Add the DOM part of the picture element, without actually using it for anything useful just yet.

BUG=368830

Review URL: https://codereview.chromium.org/261823002

git-svn-id: svn://svn.chromium.org/blink/trunk@173575 bbb929c8-8fbe-4397-9dbb-9b2b20218538