Refactor srcset parser to align it with spec changes

The srcset parser spec (http://picture.responsiveimages.org/#parse-srcset-attr )
have recently changed to have better future compatibility.
It will enable us to add future descriptors of the form "descriptor(value, anotherValue)".
It will also ease the envisioned addition of the 'h' descriptor.

BUG=376726

Review URL: https://codereview.chromium.org/293423002

git-svn-id: svn://svn.chromium.org/blink/trunk@175080 bbb929c8-8fbe-4397-9dbb-9b2b20218538