Add CustomElementState for Custom Elements v1

This patch adds CustomElementState[1] for Custom Elements v1.

In order for v0 and v1 to co-exist and avoid complexity, the state is
stored separately from V0CustomElementState.

They can be set independently from each other, except DCHECK to ensure
an element can be customized by only by one of them.

[1] https://dom.spec.whatwg.org/#concept-element-custom-element-state

BUG=594918

Review-Url: https://codereview.chromium.org/1982973002
Cr-Commit-Position: refs/heads/master@{#394734}
4 files changed