Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider.h"
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/macros.h"
@@ skipping 51 matching lines @@
   bool has_crash_logs_;
   bool has_uploaded_crash_reports_in_background_;
   bool received_memory_warning_before_last_shutdown_;
 
   DISALLOW_COPY_AND_ASSIGN(MobileSessionShutdownMetricsProviderForTesting);
 };
 
 class MobileSessionShutdownMetricsProviderTest
     : public testing::TestWithParam<int> {
  public:
-  MobileSessionShutdownMetricsProviderTest()
-      : task_runner_(new base::TestSimpleTaskRunner) {
-    base::SetRecordActionTaskRunner(task_runner_);
+  MobileSessionShutdownMetricsProviderTest() {
     metrics::MetricsService::RegisterPrefs(local_state_.registry());
   }
 
  protected:
   TestingPrefServiceSimple local_state_;
   metrics::TestMetricsServiceClient metrics_client_;
   std::unique_ptr<metrics::MetricsStateManager> metrics_state_;
   std::unique_ptr<metrics::MetricsService> metrics_service_;
   std::unique_ptr<MobileSessionShutdownMetricsProviderForTesting>
       metrics_provider_;
-  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MobileSessionShutdownMetricsProviderTest);
 };
 
 // Verifies that a sample is recorded in the correct bucket of the shutdown type
 // histogram when ProvideStabilityMetrics is invoked.
 //
 // This parameterized test receives a parameter in the range [0, 32), which is
 // used to generate values for five booleans based on the binary representation
@@ skipping 80 matching lines @@
   // Now call the method under test and verify exactly one sample is written to
   // the expected bucket.
   metrics_provider_->ProvideInitialStabilityMetrics(nullptr);
   histogram_tester.ExpectUniqueSample("Stability.MobileSessionShutdownType",
                                       expected_buckets[GetParam()], 1);
 }
 
 INSTANTIATE_TEST_CASE_P(/* No InstantiationName */,
                         MobileSessionShutdownMetricsProviderTest,
                         testing::Range(0, 32));