Revert of Have Position deal with more references (https://codereview.chromium.org/188033005/)

Reason for revert:
null can be passed to positionAfterNode() so we cannot make the argument a reference.
See crbug.com/350968.

Original issue's description:
> Have Position deal with more references
> 
> Have Position deal with more references instead of pointers. This makes the
> code look safer.
> 
> R=adamk
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168675

TBR=adamk@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/189773005

git-svn-id: svn://svn.chromium.org/blink/trunk@168854 bbb929c8-8fbe-4397-9dbb-9b2b20218538