https://bugs.webkit.org/show_bug.cgi?id=91251

Reviewed by Adam Barth.

Currently V8 String wrappers are not marked Independent.
By marking them Independent, they can be reclaimed by the scavenger GC.
Although I couldn't find a case where this change reduces memory usage,
this change would be important for upcoming changes in string conversion
between V8 and WebKit (https://bugs.webkit.org/show_bug.cgi?id=91850).

'm_lastStringImpl = 0' in StringCache::remove() is important.
Look at the following code:

    static void cachedStringCallback(v8::Persistent<v8::Value> wrapper, void* parameter)
    {
        ...;
        stringCache()->remove(stringImpl);
        wrapper.Dispose();
    }

    void StringCache::remove(StringImpl* stringImpl)
    {
        ...
        if (m_lastStringImpl.get() == stringImpl)
            m_lastStringImpl = 0;
    }

    v8::Local<v8::String> v8ExternalString(StringImpl* stringImpl, v8::Isolate* isolate)
    {
        if (m_lastStringImpl.get() == stringImpl) {
            return v8::Local<v8::String>::New(m_lastV8String); // m_lastV8String points to a wrapper object that was accessed most recently.
        }
        return v8ExternalStringSlow(stringImpl, isolate);
    }

Without 'm_lastStringImpl = 0', already disposed m_lastV8String can be used
in v8ExternalString(). This was a cause of the crashes of r122614.

Tests: At the initial commit of this patch (r122614),
       the following tests had been broken due to missing 'm_lastStringImpl = 0'.
       fast/workers/worker-location.html
       Dromaeo/cssquery-jquery.html
       Dromaeo/jslib-event-jquery.html
       Dromaeo/jslib-style-jquery.html
       Dromaeo/jslib-style-prototype.html

* bindings/v8/V8Binding.cpp:
(WebCore::StringCache::remove):
(WebCore::StringCache::v8ExternalStringSlow):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@123500 268f45cc-cd09-0410-ab3c-d52691b4dbfc