Remove the composite attribute on <img>

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/bLlTQWlLTOM/ZPrSv4t_WxUJ

BUG=428636

Review URL: https://codereview.chromium.org/688743002

git-svn-id: svn://svn.chromium.org/blink/trunk@184644 bbb929c8-8fbe-4397-9dbb-9b2b20218538