Extract ScopedTestNSSDB from nss_util.

Before ScopedTestNSSDB affected several slot getters from nss_util.h .
This change reduces ScopedTestNSSDB to solely setup a temporary test DB and not influencing the global state in nss_util anymore.

As a replacement for some of its old behavior, a new ScopedTestSystemNSSKeySlot is added, which allows to override the slot returned by GetSystemNSSKeySlot().

With this change it's now possible to write tests that need both a user and system NSS DB by using ScopedTestSystemNSSKeySlot.

As a side-effect, GetPersistentNSSKeySlot() is now compiled on !OS_CHROMEOS only.

BUG=210525
(For include changes:)

R=rsleevi@chromium.org
TBR=nkostylev@chromium.org, stevenjb@chromium.org

Review URL: https://codereview.chromium.org/401623006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285881 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/chromeos/login/auth/parallel_authenticator_unittest.cc[diff]
• chrome/browser/chromeos/net/cert_verify_proc_chromeos_unittest.cc[diff]
• chrome/browser/chromeos/policy/policy_cert_verifier_browsertest.cc[diff]
• chrome/chrome_browser.gypi[diff]
• chrome/chrome_tests.gypi[diff]
• chrome/chrome_tests_unit.gypi[diff]
• chrome/common/net/x509_certificate_model_unittest.cc[diff]
• chromeos/cert_loader_unittest.cc[diff]
• chromeos/chromeos.gyp[diff]
• chromeos/network/client_cert_resolver_unittest.cc[diff]
• chromeos/network/network_cert_migrator_unittest.cc[diff]
• chromeos/network/network_connection_handler_unittest.cc[diff]
• chromeos/network/onc/onc_certificate_importer_impl_unittest.cc[diff]
• crypto/BUILD.gn[diff]
• crypto/crypto.gyp[diff]
• crypto/nss_util.cc[diff]
• crypto/nss_util.h[diff]
• crypto/nss_util_internal.h[diff]
• crypto/rsa_private_key_nss_unittest.cc[diff]
• crypto/scoped_test_nss_chromeos_user.cc[Added - diff]
• crypto/scoped_test_nss_chromeos_user.h[Added - diff]
• crypto/scoped_test_nss_db.cc[Added - diff]
• crypto/scoped_test_nss_db.h[Added - diff]
• crypto/scoped_test_system_nss_key_slot.cc[Added - diff]
• crypto/scoped_test_system_nss_key_slot.h[Added - diff]
• net/BUILD.gn[diff]
• net/base/keygen_handler_unittest.cc[diff]
• net/cert/nss_cert_database_chromeos_unittest.cc[diff]
• net/cert/nss_cert_database_unittest.cc[diff]
• net/cert/nss_profile_filter_chromeos_unittest.cc[diff]
• net/net.gyp[diff]
• net/ssl/client_cert_store_chromeos_unittest.cc[diff]