Moved image-set tests expected results to html

Following multiple image-set image test failures and rebaselines, and most recently https://codereview.chromium.org/53093005/
I've moved their test expected results to HTML, which will hopefully lead to less flakiness.
At the very least it leads to less platform specific expectations.

Other:
* Removed lines related to flakiness of these tests from TestExpectations
* Modified the name of the green-square image to match its real dimensions
* Moved all these tests to use srcset-helper.js, instead of re-writing the same JS code in each one.

BUG=

Review URL: https://codereview.chromium.org/54673007

git-svn-id: svn://svn.chromium.org/blink/trunk@161234 bbb929c8-8fbe-4397-9dbb-9b2b20218538