https://dom.spec.whatwg.org/#dom-document-createelement
    Definition is checked for HTML namespace elements in document.createElement
    Customized built-in elements can now be created with the document.createElement
        NOTE: This is behind runtime enabled flag "CustomElementsBuiltin".

Syntax:
    https://html.spec.whatwg.org/multipage/scripting.html#custom-elements-customized-builtin-example

Implemented create element algorithm
    https://dom.spec.whatwg.org/#concept-create-element
Steps 5-7:
    /core/dom/custom/CustomElement.cpp

Added some tests.

(Sorry this patch is big)

BUG=648828,657583

Review-Url: https://codereview.chromium.org/2477713003
Cr-Commit-Position: refs/heads/master@{#433146}
7 files changed