Fix fast/css/mask-missing-image-crash.html, which became flaky by r162802.

The test has <object>s. We need to force to run post layout tasks to
show their content.

BUG=322348
TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/93873002

git-svn-id: svn://svn.chromium.org/blink/trunk@162810 bbb929c8-8fbe-4397-9dbb-9b2b20218538