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/776237Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Cr-Commit-Position: refs/heads/master@{#517852}

third_party/WebKit/LayoutTests/external/wpt/html/dom/reflection-tabular-expected.txt

 This is a testharness.js-based test.
-Found 4894 tests; 4882 PASS, 12 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 4894 tests; 4884 PASS, 10 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS table.title: 32 tests
 PASS table.lang: 32 tests
 PASS table.dir: 62 tests
@@ -113,9 +113,7 @@ NOTRUN test
 PASS td.rowSpan: 3 tests
 NOTRUN test
 PASS td.rowSpan: 4 tests
-PASS td.headers: 29 tests
-NOTRUN test
-PASS td.headers: 2 tests
+PASS td.headers: 32 tests
 PASS td.scope: 72 tests
 PASS td.abbr: 32 tests
 PASS td.align: 32 tests
@@ -145,9 +143,7 @@ NOTRUN test
 PASS th.rowSpan: 3 tests
 NOTRUN test
 PASS th.rowSpan: 4 tests
-PASS th.headers: 29 tests
-NOTRUN test
-PASS th.headers: 2 tests
+PASS th.headers: 32 tests
 PASS th.scope: 72 tests
 PASS th.abbr: 32 tests
 PASS th.align: 32 tests

third_party/WebKit/LayoutTests/fast/dom/element-attribute-js-null-expected.txt

@@ -241,7 +241,7 @@ TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.ax
 TEST SUCCEEDED: The value was the empty string. [tested HTMLTableCellElement.bgColor]
 TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.ch]
 TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.chOff]
-TEST SUCCEEDED: The value was the empty string. [tested HTMLTableCellElement.headers]
+TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.headers]
 TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.height]
 TEST SUCCEEDED: The value was the empty string. [tested HTMLTableCellElement.scope]
 TEST SUCCEEDED: The value was the string 'null'. [tested HTMLTableCellElement.vAlign]

third_party/WebKit/LayoutTests/fast/dom/element-attribute-js-null.html

@@ -565,7 +565,7 @@
                         {name: 'bgColor', expectedNull: ''},
                         {name: 'ch', expectedNull: 'null'},
                         {name: 'chOff', expectedNull: 'null'},
-                        {name: 'headers', expectedNull: ''},
+                        {name: 'headers', expectedNull: 'null'},
                         {name: 'height', expectedNull: 'null'},
                         {name: 'scope', expectedNull: ''},
                         {name: 'vAlign', expectedNull: 'null'},

third_party/WebKit/Source/core/html/HTMLTableCellElement.idl

@@ -25,7 +25,7 @@ interface HTMLTableCellElement : HTMLElement {
     [CEReactions] attribute unsigned long rowSpan;
     // TODO(foolip): headers should be a [PutForwards=value] readonly attribute
     // DOMTokenList.
-    [CEReactions, Reflect, TreatNullAs=NullString] attribute DOMString headers;
+    [CEReactions, Reflect] attribute DOMString headers;
     readonly attribute long cellIndex;
 
     // obsolete members