Implement step 1 of "look up a custom element definition"

Per [1], elements with a non-HTML namespace should cause a null
custom element definition to be returned.

Implement this step.

[1] https://html.spec.whatwg.org/multipage/custom-elements.html#look-up-a-custom-element-definition

Bug: 882773
Change-Id: Idc8f4c61b94af848e13acf4ff06da2c5f6842454
Reviewed-on: https://chromium-review.googlesource.com/1221325
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#590949}
3 files changed