Switch BrowserContextAdapter to QSharedPointer

BrowserContextAdapter is not a form of shared data, and using
QExplicitSharedDataPointer on it as always been a misuse of the class.

Instead we should switch it to QSharedPointer, which also allows us to
use QWeakPointer.

Change-Id: I8eb489b4a12d3fdddcde55821be294814a156a9d
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@theqtcompany.com>