Add preloader support for picture based source selection

This CL adds preloader support to the source selection algorithm used by HTMLImageElement when it has a picture parent.

BUG=368830

Review URL: https://codereview.chromium.org/265763010

git-svn-id: svn://svn.chromium.org/blink/trunk@174711 bbb929c8-8fbe-4397-9dbb-9b2b20218538