Remove [TreatNullAs=NullString] from Document.domain

Remove [TreatNullAs=NullString] from Document.domain

Even before this change, setting document.domain=null would fail, but
because it was treated as an empty string, where now it is "null".

BUG=460722
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196635

[philipj_slow, 2015-06-05 20:00:39]

The CQ bit was checked by philipj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-05 20:01:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1162933003/1

[commit-bot: I haz the power, 2015-06-05 21:48:28]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-05 21:48:29]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-05 21:48:31]

philipj@opera.com changed reviewers:
+ mkwst@chromium.org

[philipj_slow, 2015-06-05 21:48:31]

PTAL

[Mike West, 2015-06-07 13:46:58]

LGTM.

[philipj_slow, 2015-06-07 19:10:58]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-06-07 19:11:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1162933003/1

[commit-bot: I haz the power, 2015-06-07 20:34:41]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196635