Rework margin-collapsing for self-collapsing blocks
Refactor the way self-collapsing blocks with clearance are positioned so that they
get the correct logical-top position during margin-collapsing.

See also webkit.org/b/119979
BUG=309294

Review URL: https://codereview.chromium.org/75133007

git-svn-id: svn://svn.chromium.org/blink/trunk@163454 bbb929c8-8fbe-4397-9dbb-9b2b20218538