Move the thread hop for UMA user actions from content:: to base::.

ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm

Index: ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm
diff --git a/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm b/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm
index 0e3955ab1a9869d3d978e594c092341c9358353f..6efc02fd9a01f20c446db7f4415aa59565fa00c6 100644
--- a/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm
+++ b/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm
@@ -8,7 +8,9 @@
 
 #include "base/bind.h"
 #include "base/macros.h"
+#include "base/metrics/user_metrics.h"
 #include "base/test/histogram_tester.h"
+#include "base/test/test_simple_task_runner.h"
 #include "components/metrics/metrics_pref_names.h"
 #include "components/metrics/metrics_service.h"
 #include "components/metrics/metrics_state_manager.h"
@@ -71,7 +73,9 @@ class MobileSessionShutdownMetricsProviderForTesting
 class MobileSessionShutdownMetricsProviderTest
     : public testing::TestWithParam<int> {
  public:
-  MobileSessionShutdownMetricsProviderTest() {
+  MobileSessionShutdownMetricsProviderTest()
+      : task_runner_(new base::TestSimpleTaskRunner) {
+    base::SetRecordActionTaskRunner(task_runner_);
     metrics::MetricsService::RegisterPrefs(local_state_.registry());
   }
 
@@ -82,6 +86,7 @@ class MobileSessionShutdownMetricsProviderTest
   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);