Test that only the right resource is loaded in iframes.

Adding a couple of tests to make sure that the right resource is loaded when srcset is inside an iframe, and that *only* the right resource is loaded.

BUG=425909

Review URL: https://codereview.chromium.org/674163002

git-svn-id: svn://svn.chromium.org/blink/trunk@184373 bbb929c8-8fbe-4397-9dbb-9b2b20218538