Would be faster to not use a thread specific to implement StringImpl::empty()

Reviewed by Darin Adler, Geoffrey Garen, Maciej Stachowiak.

JavaScriptCore: 

Change JSC::UStringImpl's implementation of empty() match to match StringImpl's new implementation
(use a static defined within the empty() method), and change the interface to match too (return
a pointer not a reference). 

~0% performance impact (possible minor progression from moving empty() from .h to .cpp).

* JavaScriptCore.exp:
* runtime/Identifier.cpp:
(JSC::Identifier::add):
(JSC::Identifier::addSlowCase):
* runtime/PropertyNameArray.cpp:
(JSC::PropertyNameArray::add):
* runtime/UString.cpp:
(JSC::initializeUString):
(JSC::UString::UString):
* runtime/UStringImpl.cpp:
(JSC::UStringImpl::empty):
(JSC::UStringImpl::create):
* runtime/UStringImpl.h:
(JSC::UStringImpl::adopt):
(JSC::UStringImpl::createUninitialized):
(JSC::UStringImpl::tryCreateUninitialized):

WebCore: 

Copy JavaScriptCore in making 'static' strings threadsafe, make the empty string a static,
shared by all threads.

~2% progression on Dromaeo DOM core & JS lib tests.

* platform/ThreadGlobalData.cpp:
(WebCore::ThreadGlobalData::ThreadGlobalData):
(WebCore::ThreadGlobalData::~ThreadGlobalData):
* platform/ThreadGlobalData.h:
(WebCore::ThreadGlobalData::eventNames):
* platform/text/StringImpl.cpp:
(WebCore::StringImpl::StringImpl):
(WebCore::StringImpl::empty):
* platform/text/StringImpl.h:
(WebCore::StringImpl::deref):
(WebCore::StringImpl::hasOneRef):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@55825 268f45cc-cd09-0410-ab3c-d52691b4dbfc