Split the fast/hidpi/image-srcset-png.html test up to avoid scrollbars.

This test has 4 test cases but only 2 are visible and end up being
tested. This makes the bottom 2 cases not useful, and also introduces
scrollbars to this test which we don't need or want, as they cause
pixel differences when composited.

Split the test up into each of the 4 test cases individually.

R=enne@chromium.org
BUG=364946

Review URL: https://codereview.chromium.org/247123004

git-svn-id: svn://svn.chromium.org/blink/trunk@172166 bbb929c8-8fbe-4397-9dbb-9b2b20218538