Reviewed by Darin Adler

        https://bugs.webkit.org/show_bug.cgi?id=21818

        Add HashTraits for AtomicString so that AtomicString can be used as
        the key for a HashMap or HashSet.

        * GNUmakefile.am: add AtomicStringHash.h
        * WebCore.vcproj/WebCore.vcproj: add AtomicStringHash.h
        * WebCore.xcodeproj/project.pbxproj: add AtomicStringHash.h
        * platform/text/AtomicString.h:
        (WebCore::AtomicString::AtomicString):
        (WebCore::AtomicString::isHashTableDeletedValue):
            specialize DefaultHash for AtomicString to use AtomicStringHash
        * platform/text/AtomicStringHash.h: Added.
        * platform/text/StringImpl.cpp:
        (WebCore::StringImpl::StringImpl):
            compute the hash up-front for the empty string
        * platform/text/StringImpl.h:
        (WebCore::StringImpl::existingHash):
            method to get the hash without a test and branch, for callers like
            AtomicStringHash who can guarantee that the hash has already been computed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37852 268f45cc-cd09-0410-ab3c-d52691b4dbfc