Fix srcset selection bug when 'w' and src are present

When srcset had 'w' based resources that are all smaller than their display size imposed by 'sizes',
src got picked, contrary to expected behavior.
This CL fixes that and adds regression tests for that case.

BUG=365787

Review URL: https://codereview.chromium.org/249353002

git-svn-id: svn://svn.chromium.org/blink/trunk@172458 bbb929c8-8fbe-4397-9dbb-9b2b20218538