Allow using quota management API for non-standard URLs

The DCHECK in quota_manager.cc ensures that only the standard origins
can request persistent storage. This patch removes the DCHECK and allows
qrc:/ and custom URL scheme handlers to use quotas.

Change-Id: I68a5915d66599fbb3ea844fe1ec3f0b646cbc7b8
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>