HTMLTableHeaderCellElement.scope should only return known values

HTMLTableHeaderCellElement.scope should only return known values
https://bugs.webkit.org/show_bug.cgi?id=154423
<rdar://problem/24731018>

Reviewed by Ryosuke Niwa.

LayoutTests/imported/w3c:

Rebaseline now that more checks are passing.

* web-platform-tests/html/dom/reflection-tabular-expected.txt:

Source/WebCore:

HTMLTableHeaderCellElement.scope should only return known values as per:
- https://html.spec.whatwg.org/multipage/tables.html#dom-th-scope

Known values are document here:
- https://html.spec.whatwg.org/multipage/tables.html#attr-th-scope

No new tests, already covered by existing test.

* CMakeLists.txt:
* WebCore.vcxproj/WebCore.vcxproj:
* WebCore.vcxproj/WebCore.vcxproj.filters:
* WebCore.xcodeproj/project.pbxproj:
* html/HTMLElementsAllInOne.cpp:
* html/HTMLTableHeaderCellElement.cpp: Copied from Source/WebCore/html/HTMLTableHeaderCellElement.h.
(WebCore::HTMLTableHeaderCellElement::scope):
(WebCore::HTMLTableHeaderCellElement::setScope):
* html/HTMLTableHeaderCellElement.h:
* html/HTMLTableHeaderCellElement.idl:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@196793 268f45cc-cd09-0410-ab3c-d52691b4dbfc