Remove an unnecessary layout when a positioned element's margin changes

Postioned objects' margins don't collapse so do not depend on the
collapsing code in their parents to position them properly. Remove this
unnecessary layout on the parent.

BUG=332860

Review URL: https://codereview.chromium.org/571803002

git-svn-id: svn://svn.chromium.org/blink/trunk@182116 bbb929c8-8fbe-4397-9dbb-9b2b20218538