Add a basic test for main frame swap.

This doesn't really invoke all the machinery involved in swapping
frames (a real frame swap would likely have a provisional local
frame before it committed into the frame tree, etc.) but it's
still helpful for catching problematic code.

BUG=346764

Review URL: https://codereview.chromium.org/409823002

git-svn-id: svn://svn.chromium.org/blink/trunk@183840 bbb929c8-8fbe-4397-9dbb-9b2b20218538