Implement PotentialCustomElementName

This patch implements PotentialCustomElementName, which is part of
validating custom element name.

A valid custom element name must be PotentialCustomElementName and also
exclude all hyphen-containing element names. It will be in a following
patch.

Spec:
https://html.spec.whatwg.org/multipage/scripting.html#prod-potentialcustomelementname

BUG=594918

Review URL: https://codereview.chromium.org/1916013004

Cr-Commit-Position: refs/heads/master@{#390039}
10 files changed