Editing: Let Selection.collapse with null clears selection.

The current spec says Selection.collapse(null) raises exception and the 
implementation follows.
However, many web apps assume that chrome clears selection when 
Selection.collapse(null) calls.
Thus we revert the behavior.

BUG=395318

Review URL: https://codereview.chromium.org/546703003

git-svn-id: svn://svn.chromium.org/blink/trunk@181487 bbb929c8-8fbe-4397-9dbb-9b2b20218538