Thead setter should throw if not handed a thead parameter

Thead setter should throw if not handed a thead parameter [1]:
"If the new value is neither null nor a thead element, then
a "HierarchyRequestError" DOMException must be thrown instead."

[1] https://html.spec.whatwg.org/#dom-table-thead

Bug: 651572
Change-Id: Ia65a259b7bc6e9cca50d5491c497012149d1fa8e
Reviewed-on: https://chromium-review.googlesource.com/721860
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509588}
3 files changed