Implement CustomElement::isValidName()

This patch implements CustomElement::IsValidName().

A valid custom element name is PotentialCustomElementName except all
hyphen-containing element names from the applicable specifications,
according to the spec below:
https://html.spec.whatwg.org/multipage/scripting.html#valid-custom-element-name

BUG=594918

Review-Url: https://codereview.chromium.org/1914383002
Cr-Commit-Position: refs/heads/master@{#390308}
3 files changed