Mark fast/images/image-zoom-to-25.html as flaky

Unreviewed gardening.

TBR=tony@chromium.org
BUG=307570

Review URL: https://codereview.chromium.org/27240003

git-svn-id: svn://svn.chromium.org/blink/trunk@159680 bbb929c8-8fbe-4397-9dbb-9b2b20218538