Gardening: image-zoom-to-25.html is flaky.

This test has been flaky since birth in r156402 on Linux.

BUG=276759
TBR=tony@chromium.org

Review URL: https://codereview.chromium.org/23004007

git-svn-id: svn://svn.chromium.org/blink/trunk@156435 bbb929c8-8fbe-4397-9dbb-9b2b20218538