Make headers td.headers accept null as string

Nothing in the spec [1] mentions having to convert JS null to a
null/empty string internally and the wpt test indicates we should
just do the default, i.e. stringify to "null", so remove the
TreatNullAs extended attribute.

Behavior matches Firefox and Edge.

[1] https://html.spec.whatwg.org/#dom-tdth-headers

Bug: 651762

Change-Id: I400d86bfa19702cef4607c2ae0d21a2becf7ee0a
Reviewed-on: https://chromium-review.googlesource.com/776237
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Cr-Commit-Position: refs/heads/master@{#517852}
4 files changed