DeleteInstanceForTesting method for the DesktopSessionDurationTracker

This method deletes the instance and resets it to nullptr to avoid
memory leaks in tests.

Bug: 743049
Change-Id: Ib14253155c2282dd8c4cedcc9bc25f7389ae7a28
Reviewed-on: https://chromium-review.googlesource.com/571504Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Catherine Chung <catherinechung@google.com>
Cr-Commit-Position: refs/heads/master@{#487994}

chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.cc

@@ -21,6 +21,7 @@ const base::TimeDelta kZeroTime = base::TimeDelta::FromSeconds(0);
 
 // static
 void DesktopSessionDurationTracker::Initialize() {
+  DCHECK(!g_instance);
   g_instance = new DesktopSessionDurationTracker;
 }
 
@@ -75,6 +76,13 @@ void DesktopSessionDurationTracker::OnUserEvent() {
   }
 }
 
+// static
+void DesktopSessionDurationTracker::CleanupForTesting() {
+  DCHECK(g_instance);
+  delete g_instance;
+  g_instance = nullptr;
+}
+
 void DesktopSessionDurationTracker::OnAudioStart() {
   // This may start session.
   is_audio_playing_ = true;

chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.h

@@ -58,6 +58,9 @@ class DesktopSessionDurationTracker : public AudibleContentsTracker::Observer {
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
+  // Cleans up any global state for testing.
+  static void CleanupForTesting();
+
  protected:
   DesktopSessionDurationTracker();
   ~DesktopSessionDurationTracker() override;

chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker_unittest.cc

@@ -70,6 +70,14 @@ class DesktopSessionDurationTrackerTest : public testing::Test {
   DesktopSessionDurationTrackerTest()
       : loop_(base::MessageLoop::TYPE_DEFAULT) {}
 
+  void SetUp() override {
+    metrics::DesktopSessionDurationTracker::Initialize();
+  }
+
+  void TearDown() override {
+    metrics::DesktopSessionDurationTracker::CleanupForTesting();
+  }
+
   void ExpectTotalDuration(base::TimeDelta duration) {
     histogram_tester_.ExpectTotalCount("Session.TotalDuration", 1);
     base::Bucket bucket =