Gardening: Mark http/tests/misc/image-blocked-src-change.html as flaky

TBR=jchaffraix@chromium.org
BUG=377970

Review URL: https://codereview.chromium.org/300113006

git-svn-id: svn://svn.chromium.org/blink/trunk@174890 bbb929c8-8fbe-4397-9dbb-9b2b20218538