QQmlContextData gets cleared too early

When a XMLHttpRequest is performed from within the calling context of an external script, a reference to the context is created by QXMLHttpRequest and stored in a non-persistent v8 variable.

When this variable is garbage collected after the XMLHttpRequest completes, QV8ContextResource clears the associated QQmlContextData and no further XMLHttpRequests are possible from this context.

A patch for this issue is available at https://codereview.qt-project.org/41481