[MultiDeviceSetup] Make AndroidSmsPairingStateTracker observers checked.

This CL makes AndroidSmsPairingStateTracker observers checked. This will
prevent us from notifying destroyed observers.

Bug: 926429
Change-Id: I6bffbf3a6fd7eb3e1bd59a640a851a733057e229
Reviewed-on: https://chromium-review.googlesource.com/c/1449170
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628475}

chromeos/services/multidevice_setup/public/cpp/android_sms_pairing_state_tracker.h

@@ -14,12 +14,12 @@ namespace multidevice_setup {
 // Inspects and track pairing state of the Messages for Web PWA.
 class AndroidSmsPairingStateTracker {
  public:
-  class Observer {
+  class Observer : public base::CheckedObserver {
    public:
     virtual void OnPairingStateChanged() = 0;
 
    protected:
-    virtual ~Observer() = default;
+    ~Observer() override = default;
   };
 
   AndroidSmsPairingStateTracker();
@@ -35,7 +35,7 @@ class AndroidSmsPairingStateTracker {
   void NotifyPairingStateChanged();
 
  private:
-  base::ObserverList<Observer>::Unchecked observer_list_;
+  base::ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(AndroidSmsPairingStateTracker);
 };