Add a test case for fallback content in the object element.

Rendering of these test cases is very inconsistent among browsers so if
we ever decide to harmonize this is our baseline.

BUG=2210

Review URL: https://codereview.chromium.org/250673002

git-svn-id: svn://svn.chromium.org/blink/trunk@172662 bbb929c8-8fbe-4397-9dbb-9b2b20218538