Note that DOM says attribute namespaces are null, or a non-empty string:

https://dom.spec.whatwg.org/#concept-attribute-namespace

Chrome passes an empty string, not null, when there is no namespace.

This causes a number of WebKit's tests to fail: LayoutTests/fast/custom-elements/attribute-changed-callback.html.

I notice we have to tests for namespaced attributes, either.

Patch up at https://codereview.chromium.org/2303663004

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d39405eee528d5cec4edcb6dd9abfdc83b897fe8

commit d39405eee528d5cec4edcb6dd9abfdc83b897fe8
Author: dominicc <dominicc@chromium.org>
Date: Sun Sep 04 00:11:52 2016

Pass null to attributeChangedCallback when there's no namespace URI

The DOM spec says attribute namespaces may be null, or non-empty [1].
We should accurately transmit those nulls to the
attributeChangedCallback [2]. We were converting null to empty string.

This also adds a test that non-null namespaces are transmitted, too.

[1] https://dom.spec.whatwg.org/#concept-attribute-namespace
[2] https://dom.spec.whatwg.org/#concept-element-attributes-change , etc.

BUG= 643046 

Review-Url: https://codereview.chromium.org/2303663004
Cr-Commit-Position: refs/heads/master@{#416458}

[modify] https://crrev.com/d39405eee528d5cec4edcb6dd9abfdc83b897fe8/third_party/WebKit/LayoutTests/custom-elements/spec/callback.html
[modify] https://crrev.com/d39405eee528d5cec4edcb6dd9abfdc83b897fe8/third_party/WebKit/LayoutTests/custom-elements/spec/upgrade-element.html
[modify] https://crrev.com/d39405eee528d5cec4edcb6dd9abfdc83b897fe8/third_party/WebKit/Source/bindings/core/v8/ScriptCustomElementDefinition.cpp