Gardening: image-zoom-to-500.html is also flaky on Linux.

BUG=276759
TBR=tony@chromium.org

Review URL: https://codereview.chromium.org/22866016

git-svn-id: svn://svn.chromium.org/blink/trunk@156437 bbb929c8-8fbe-4397-9dbb-9b2b20218538