Change BrowserContextAdapter to be QPointer

BrowserContextAdapter is living and accessed only in UI thread,
remove QSharedPointer usage and change QWebengineProfile to use
QPointer. Prevent BrowserContextAdapter outliving WebEngineContext
by setting globalObject as a parent to track WebEngineContext
destruction.

This commits tries to simplify the life cycle handling
of browser context, it removes profile shutdown
methods and QWebEngineBrowserContext, which was used
to track profile destruction.

Task-number: QTBUG-62147
Change-Id: I79f2c38a123cd053e3a59f4900afbdc759a396fe
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>