Test srcset parsing for space in url

A false bug report led me to the realization that srcset's unit testing didn't include URLs with encoded spaces in them.
Added such a test here.

BUG=422914

Review URL: https://codereview.chromium.org/643703005

git-svn-id: svn://svn.chromium.org/blink/trunk@183858 bbb929c8-8fbe-4397-9dbb-9b2b20218538