Mark wpt/largest-contentful-paint/observe-image.html as flaky

Bug: 978587
Change-Id: I595817af2bb211c84b8557eb4f2c01aedf22140a
Tbr: npm@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1677625
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672346}
1 file changed