Make createAttributeNS's qualifiedName argument non-nullable

Make createAttributeNS's qualifiedName argument non-nullable

Before this change, document.createAttributeNS("foo", null) would throw
"Failed to execute 'createAttributeNS' on 'Document': The qualified name
provided is empty." because the null string was treated as empty.

After this change, document.createAttributeNS("foo", null) will create
an Attr object with localName "null", which matches Firefox and IE.

Usage of this API is extremely low, making the risk equally so:
https://www.chromestatus.com/metrics/feature/timeline/popularity/112

BUG=460722
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196940

[philipj_slow, 2015-06-11 10:23:24]

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

[commit-bot: I haz the power, 2015-06-11 10:23:46]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1164123003/20001

[commit-bot: I haz the power, 2015-06-11 11:42:03]

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

[commit-bot: I haz the power, 2015-06-11 11:42:04]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-11 11:44:06]

philipj@opera.com changed reviewers:
+ jl@opera.com

[philipj_slow, 2015-06-11 11:44:07]

PTAL

[Jens Widell, 2015-06-11 11:46:17]

LGTM

[philipj_slow, 2015-06-11 11:49:41]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-06-11 11:50:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1164123003/20001

[commit-bot: I haz the power, 2015-06-11 11:53:41]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196940