Add ASSERTS to document when postLayoutTasks occur

Turns out they occur slightly more often than
even I thought, so hopefully these ASSERTs will
help others in this code to better understand
the order of things.

Review URL: https://codereview.chromium.org/194343002

git-svn-id: svn://svn.chromium.org/blink/trunk@168960 bbb929c8-8fbe-4397-9dbb-9b2b20218538