Fix tracking of the id attribute string if it is shared across elements.

The patch to remove AtomicStringImpl:
http://src.chromium.org/viewvc/blink?view=rev&rev=154790

Exposed a lifetime issue with strings for id attributes. We simply need to use
AtomicString.

BUG=290566

Review URL: https://codereview.chromium.org/33793004

git-svn-id: svn://svn.chromium.org/blink/trunk@160250 bbb929c8-8fbe-4397-9dbb-9b2b20218538
inline v8::Handle<v8::Value> DialogHandler::returnValue(v8::Isolate* isolate) const
{
    if (m_dialogContext.IsEmpty())
        return v8::Undefined(isolate);
    v8::Context::Scope scope(m_dialogContext);
    v8::Handle<v8::Value> returnValue = m_dialogContext->Global()->Get(v8::String::NewSymbol("returnValue"));
    if (returnValue.IsEmpty())
        return v8::Undefined(isolate);
    return returnValue;
}

inline v8::Handle<v8::Value> DialogHandler::returnValue(v8::Isolate* isolate) const
{
    if (m_dialogContext.IsEmpty())
        return v8::Undefined(isolate);
    v8::Context::Scope scope(m_dialogContext);
    v8::Handle<v8::Value> returnValue = m_dialogContext->Global()->Get(v8::String::NewSymbol("returnValue"));
    if (returnValue.IsEmpty())
        return v8::Undefined(isolate);
    return returnValue;
}
