<http://webkit.org/b/94990>

Reviewed by Antti Koivisto.

Keep a cache of ElementAttributeData objects for a given set of attributes and reuse
them in elements with identical attribute maps. ElementAttributeData is made ref-counted
to facilitate this. A copy-on-write mechanism is already in place, since mutating call
sites have to go via Element::mutableAttributeData().

The cache is held by Document and cleared in Document::finishedParsing() since the vast
majority of immutable ElementAttributeData will be constructed during parsing.

On the HTML5 spec at <http://whatwg.org/c/>, we get a cache hit rate of nearly 80%,
translating into a 3.5MB reduction in memory use.

* dom/Document.cpp:
(WebCore::Document::finishedParsing):
(ImmutableAttributeDataCacheKey):
(WebCore::ImmutableAttributeDataCacheKey::ImmutableAttributeDataCacheKey):
(WebCore::ImmutableAttributeDataCacheKey::operator!=):
(WebCore::ImmutableAttributeDataCacheKey::hash):
(ImmutableAttributeDataCacheEntry):
(WebCore::Document::cachedImmutableAttributeData):
* dom/Document.h:
* dom/Element.cpp:
(WebCore::Element::parserSetAttributes):
* dom/Element.h:
* dom/ElementAttributeData.cpp:
(WebCore::ElementAttributeData::createImmutable):
(WebCore::ElementAttributeData::ElementAttributeData):
* dom/ElementAttributeData.h:
(WebCore::ElementAttributeData::create):
(ElementAttributeData):
(WebCore::ElementAttributeData::makeMutable):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@127438 268f45cc-cd09-0410-ab3c-d52691b4dbfc