Extend srcset's syntax to include 'w' descriptors

This CL extends srcset syntax to match the picture specification.
It also adds unit tests to the srcset parser.

BUG=357386

Review URL: https://codereview.chromium.org/233213003

git-svn-id: svn://svn.chromium.org/blink/trunk@171307 bbb929c8-8fbe-4397-9dbb-9b2b20218538