Fix srcset invalid descriptor test

The srcset invalid desriptor test broke when I changed the test resources, showing opposite results.
This CL fixes the test, while clarifying the test's purpose.

BUG=305468
NOTRY=TRUE

Review URL: https://codereview.chromium.org/26441005

git-svn-id: svn://svn.chromium.org/blink/trunk@159734 bbb929c8-8fbe-4397-9dbb-9b2b20218538