Clear the testing provider on shutdown.

Restore code to clean up the test instance of the PolicyProvider, so unit tests
that set the PolicyProvider no longer crash when run in succession.

Bug: None
Test: Run unit_tests --gtest_filter=ProfilePolicyConnectorTest*
Change-Id: I3a90fb04f3736c7e1351791b52886d726913aeed
Reviewed-on: https://chromium-review.googlesource.com/893098Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Drew Wilson <atwilson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532865}

components/policy/core/browser/browser_policy_connector_base.cc

@@ -60,6 +60,9 @@ void BrowserPolicyConnectorBase::Shutdown() {
     for (const auto& provider : *policy_providers_)
       provider->Shutdown();
   }
+  // Drop g_testing_provider so that tests executed with --single_process can
+  // call SetPolicyProviderForTesting() again. It is still owned by the test.
+  g_testing_provider = nullptr;
   g_created_policy_service = false;
 }