Ship HTMLImageElement's srcset attribute (DPR)

BUG=296891

Intent to ship thread - https://groups.google.com/a/chromium.org/d/msg/blink-dev/ONd27GqklLA/a3nG4CyC-RMJ

Review URL: https://codereview.chromium.org/151233003

git-svn-id: svn://svn.chromium.org/blink/trunk@166264 bbb929c8-8fbe-4397-9dbb-9b2b20218538