Reviewed by David Levin.

        Cache v8 strings when converting from WebCore::String to v8 string.
        https://bugs.webkit.org/show_bug.cgi?id=27655

        * bindings/v8/V8Binding.cpp:
        (WebCore::v8String): now just immediately calls v8ExternalString
        (WebCore::enableStringImplCache): enables caching of conversions from WebCore::StringImpl to
        v8::String
        (WebCore::makeExternalString): utilty function to create external v8::String out of
        WebCore::String
        (WebCore::getStringCache): static function to access string cache
        (WebCore::cachedStringCallback): callback for weak handles of v8::Strings stored in the
        cache
        (WebCore::v8ExternalString): if caching enabled, checks if there is already v8::String for
        the given WebCore::StringImpl.  If present, returns it, otherwise creates a new v8 external
        string.
        * bindings/v8/V8Binding.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@46580 268f45cc-cd09-0410-ab3c-d52691b4dbfc