Have Position deal with more references

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

[adamk, 2014-03-06 20:36:50]

lgtm

[commit-bot: I haz the power, 2014-03-06 20:36:55]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@samsung.com/188033005/1

[commit-bot: I haz the power, 2014-03-06 20:48:36]

Change committed as 168675

[Inactive, 2014-03-10 19:17:21]

A revert of this CL has been created in
https://codereview.chromium.org/189773005/ by ch.dumez@samsung.com.

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