Have srcset respond to viewport changes

This CL gives srcset+sizes based images the ability to react to viewport changes, by selecting a more fitting resource, and by modifying their intrinsic sizing according to the viewport.

BUG=392474

Review URL: https://codereview.chromium.org/369423002

git-svn-id: svn://svn.chromium.org/blink/trunk@179422 bbb929c8-8fbe-4397-9dbb-9b2b20218538