Change dynamic srcset test to avoid useless download

In order to avoid useless downloads when dynamically adding a srcset attribute, the attribute's value must include the 1x resource as well.
I've changed the dynamic srcset addition test to reflect that (and stop downloading the 2x resource for nothing)

BUG=306523

Review URL: https://codereview.chromium.org/26849003

git-svn-id: svn://svn.chromium.org/blink/trunk@159768 bbb929c8-8fbe-4397-9dbb-9b2b20218538