Fix "unused variable" warnings.

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 37 matching lines @@
   bool HasCrashLogs() override { return has_crash_logs_; }
   bool HasUploadedCrashReportsInBackground() override {
     return has_uploaded_crash_reports_in_background_;
   }
   bool ReceivedMemoryWarningBeforeLastShutdown() override {
     return received_memory_warning_before_last_shutdown_;
   }
 
  private:
   bool is_first_launch_after_upgrade_;
-  bool was_last_shutdown_clean_;
   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:
@@ skipping 104 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));