calling whenDefined with invalid name should throw SyntaxError

Added test for customElements.whenDefined
Returns a promise rejected with a "SyntaxError" DOMException
if not given a valid custom element name.

BUG=594918

Review-Url: https://codereview.chromium.org/2100403002
Cr-Commit-Position: refs/heads/master@{#404117}
3 files changed