ShareableElementData should use zero-length array for storage.

ShareableElementData should use zero-length array for storage.
<http://webkit.org/b/109959>

Reviewed by Anders Carlsson.

Use a zero-length Attribute array instead of always casting from void* to an array.
It was done this way originally because I didn't know we could sidestep the MSVC
build error with some #pragma hackery.

* dom/DocumentSharedObjectPool.cpp:
(WebCore::DocumentSharedObjectPool::cachedShareableElementDataWithAttributes):
* dom/Element.cpp:
(WebCore::sizeForShareableElementDataWithAttributeCount):
(WebCore::ShareableElementData::ShareableElementData):
(WebCore::ShareableElementData::~ShareableElementData):
(WebCore::UniqueElementData::UniqueElementData):
* dom/Element.h:
(ShareableElementData):
(WebCore::ElementData::attributeItem):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@143044 268f45cc-cd09-0410-ab3c-d52691b4dbfc