Split NonDocumentTypeChildNode from ChildNode

Split NonDocumentTypeChildNode from ChildNode

Blink is the only engine to still expose previousElementSibling and
nextElementSibling and http://mail1.griffithscorp.com/ doesn't show a
login button as a result.

More importantly, this brings is in alignment with the spec and other
browsers.

BUG=441238
TEST=LayoutTests/fast/dom/DocumentType/previous-element-sibling.html
     LayoutTests/fast/dom/DocumentType/next-element-sibling.html
     LayoutTests/fast/dom/Element/element-traversal
     LayoutTests/fast/dom/Text/previous-element-sibling.html
     LayoutTests/fast/dom/Text/next-element-sibling.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=186969

[philipj_slow, 2014-12-11 13:42:45]

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

[philipj_slow, 2014-12-11 13:42:45]

PTAL

[arv (Not doing code reviews), 2014-12-11 16:42:55]

LGTM

[philipj_slow, 2014-12-11 16:59:57]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-12-11 17:00:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/799523002/1

[commit-bot: I haz the power, 2014-12-11 18:08:21]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=186969