Fix a srcset test.

The test was not written correctly. See the comments I added for
the explanation. With this change (and after https://codereview.chromium.org/200923002)
it now passes.

R=abarth@chromium.org,yoav@yoav.ws
BUG=341172

Review URL: https://codereview.chromium.org/281743002

git-svn-id: svn://svn.chromium.org/blink/trunk@173957 bbb929c8-8fbe-4397-9dbb-9b2b20218538