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When setting tFoot or using createTFoot the new
element should be inserted at the end.

This patch also changes behavior so we throw a
HierarchyRequestError [1] when setting tFoot with
a new element not of type tfoot.

Behavior matches Safari, Firefox and Edge.

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

BUG=641338

Review-Url: https://codereview.chromium.org/2338013007
Cr-Commit-Position: refs/heads/master@{#419468}