Add "Failed" custom element state

To avoid constructor running multiple times on the same element[1],
a new "failed" custom element state[2] was added to the spec.

This patch:
1. Adds the CustomElementState::Failed.
2. Simplifies getCustomElementState().
3. Removes Element::isCustomElement() to match code conventions to the
   spec terminologies.

[1] https://github.com/w3c/webcomponents/issues/533
[2] https://dom.spec.whatwg.org/#concept-element-custom-element-state

BUG=594918

Review-Url: https://codereview.chromium.org/2173623003
Cr-Commit-Position: refs/heads/master@{#407754}
11 files changed