Support Qt::AA_ShareOpenGLContexts

QtWebEngine::initialize happens after QGuiApplication instantiation,
so it is too late to simply set the application attribute.
In that scenario, we don't want to override the shared context and risk
a double free when both QGuiApplication and our cleanup handler try
to delete it.

Change-Id: Id52884abbb0c2380208876d9c00e8ddbfbc21eda
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>