Add HTMLPictureElement-based source selection to HTMLImageElement

This CL adds HTMLPictureElement-based source selection algorithm to 
HTMLImageElement and makes sure that the right resource is picked when
HTMLImageElement has an HTMLPictureElement parent.

BUG=368830

Review URL: https://codereview.chromium.org/288033018

git-svn-id: svn://svn.chromium.org/blink/trunk@174354 bbb929c8-8fbe-4397-9dbb-9b2b20218538