[Gardening] suppress failing image-set tests

r155689 exposed a problem which is causing the following to assert:

fast/hidpi/image-set-out-of-order.html
fast/hidpi/image-set-without-specified-width.html
virtual/gpu/fast/hidpi/image-set-out-of-order.html
virtual/gpu/fast/hidpi/image-set-without-specified-width.html

BUG=269627
TBR=pkasting@chromium.org

Review URL: https://codereview.chromium.org/22606002

git-svn-id: svn://svn.chromium.org/blink/trunk@155700 bbb929c8-8fbe-4397-9dbb-9b2b20218538