Revert of Have Position deal with more references

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168854

[Inactive, 2014-03-10 19:17:22]

Created Revert of Have Position deal with more references

[commit-bot: I haz the power, 2014-03-10 19:17:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@samsung.com/189773005/1

[Inactive, 2014-03-10 19:18:25]

The CQ bit was unchecked by ch.dumez@samsung.com

[commit-bot: I haz the power, 2014-03-10 19:18:36]

Change committed as 168854