Reviewed by David Levin.

        Inform V8 of the amount of WebCore string memory it is keeping alive.
        https://bugs.webkit.org/show_bug.cgi?id=27537

        V8 uses external strings that are backed by WebCore strings. Since
        the external strings themselves are small, V8 has no way of
        knowing how much memory it is actually holding on to. With this
        change, we inform V8 of the amount of WebCore string data it is
        holding on to with external strings.

        * bindings/v8/V8Binding.cpp:
        (WebCore::WebCoreStringResource::WebCoreStringResource):
        (WebCore::WebCoreStringResource::~WebCoreStringResource):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@46241 268f45cc-cd09-0410-ab3c-d52691b4dbfc