Revert r162802 "Plugin elements always start content loading asynchronously."
Revert r162810 "Fix fast/css/mask-missing-image-crash.html, which became flaky by r162802."

They broke some plugin-related tests in content_browsertests.

BUG=322348
TBR=timloh@chromium.org

Review URL: https://codereview.chromium.org/94003002

git-svn-id: svn://svn.chromium.org/blink/trunk@162815 bbb929c8-8fbe-4397-9dbb-9b2b20218538