Add srcset negative x descriptor value tests

The implementation handled negative x descriptors properly, but they weren't tested.

This CL fixes that.

BUG=376726
CC=simonp@opera.com

Review URL: https://codereview.chromium.org/292383004

git-svn-id: svn://svn.chromium.org/blink/trunk@175033 bbb929c8-8fbe-4397-9dbb-9b2b20218538