Do not create more than one ElementVisibilityObserver.

In some rare cases, we might have an ElementVisibilityObserver pending
and be back in a situation where we want to create one. In this case,
instead of creating another one, we can just skip the step.

BUG=624927
TEST=cluster-fuzz
R=zqzhang@chromium.org

Review-Url: https://codereview.chromium.org/2118313002
Cr-Commit-Position: refs/heads/master@{#403684}
1 file changed