SystemMonitor: Unregister the observers when they are destroyed

This will ensure that no observers are being leaked.

Change-Id: I41d20eb4105afb894423113029563a548ec8507e
Reviewed-on: https://chromium-review.googlesource.com/c/1487361Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636823}

chrome/browser/performance_monitor/system_monitor.cc

@@ -75,6 +75,13 @@ SystemMonitor* SystemMonitor::Get() {
   return g_system_metrics_monitor;
 }
 
+SystemMonitor::SystemObserver::~SystemObserver() {
+  if (g_system_metrics_monitor) {
+    // This is a no-op if the observer has already been removed.
+    g_system_metrics_monitor->RemoveObserver(this);
+  }
+}
+
 void SystemMonitor::SystemObserver::OnFreePhysicalMemoryMbSample(
     int free_phys_memory_mb) {
   NOTREACHED();
@@ -103,8 +110,8 @@ void SystemMonitor::AddOrUpdateObserver(
 void SystemMonitor::RemoveObserver(SystemMonitor::SystemObserver* observer) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   observers_.RemoveObserver(observer);
-  observer_metrics_.erase(observer);
-  UpdateObservedMetrics();
+  if (observer_metrics_.erase(observer))
+    UpdateObservedMetrics();
 }
 
 SystemMonitor::MetricVector SystemMonitor::GetMetricsToEvaluate() const {

chrome/browser/performance_monitor/system_monitor.h

@@ -77,7 +77,7 @@ class SystemMonitor {
           SamplingFrequency::kNoSampling;
     };
 
-    ~SystemObserver() override = default;
+    ~SystemObserver() override;
 
     // Reports the amount of free physical memory, in MB.
     virtual void OnFreePhysicalMemoryMbSample(int free_phys_memory_mb);
@@ -102,7 +102,8 @@ class SystemMonitor {
       SystemObserver::MetricRefreshFrequencies metrics_frequencies);
 
   // Removes |observer| from the observer list. After this call, the observer
-  // will not receive notifications for any metric.
+  // will not receive notifications for any metric. This is a no-op if this
+  // observer isn't registred.
   void RemoveObserver(SystemObserver* observer);
 
   const base::OneShotTimer& refresh_timer_for_testing() {