android: makes UsageStatsBridge properly remove observer

It looks like UsageStatsBridge is never destroyed. This ensures
the observer it adds to HistoryService is properly removed.

BUG=1158388
TEST=none

Change-Id: I8640dde7b2eaf2d5701827cf800e5c52edb605d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613587Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840831}

chrome/browser/android/usage_stats/usage_stats_bridge.cc

@@ -62,16 +62,10 @@ UsageStatsBridge::UsageStatsBridge(
       HistoryServiceFactory::GetForProfile(profile_,
                                            ServiceAccessType::IMPLICIT_ACCESS);
   if (history_service)
-    history_service->AddObserver(this);
+    scoped_history_service_observer_.Observe(history_service);
 }
 
-UsageStatsBridge::~UsageStatsBridge() {
-  history::HistoryService* history_service =
-      HistoryServiceFactory::GetForProfile(profile_,
-                                           ServiceAccessType::IMPLICIT_ACCESS);
-  if (history_service)
-    history_service->RemoveObserver(this);
-}
+UsageStatsBridge::~UsageStatsBridge() = default;
 
 void UsageStatsBridge::Destroy(JNIEnv* env, const JavaRef<jobject>& j_this) {
   delete this;
@@ -346,4 +340,9 @@ void UsageStatsBridge::OnURLsDeleted(
   }
 }
 
+void UsageStatsBridge::HistoryServiceBeingDeleted(
+    history::HistoryService* history_service) {
+  scoped_history_service_observer_.Reset();
+}
+
 }  // namespace usage_stats

chrome/browser/android/usage_stats/usage_stats_bridge.h

@@ -11,7 +11,9 @@
 
 #include "base/android/scoped_java_ref.h"
 #include "base/memory/weak_ptr.h"
+#include "base/scoped_observation.h"
 #include "chrome/browser/android/usage_stats/usage_stats_database.h"
+#include "components/history/core/browser/history_service.h"
 #include "components/history/core/browser/history_service_observer.h"
 
 namespace history {
@@ -94,6 +96,8 @@ class UsageStatsBridge : public history::HistoryServiceObserver {
   // Overridden from history::HistoryServiceObserver.
   void OnURLsDeleted(history::HistoryService* history_service,
                      const history::DeletionInfo& deletion_info) override;
+  void HistoryServiceBeingDeleted(
+      history::HistoryService* history_service) override;
 
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
 
@@ -119,6 +123,10 @@ class UsageStatsBridge : public history::HistoryServiceObserver {
 
   base::android::ScopedJavaGlobalRef<jobject> j_this_;
 
+  base::ScopedObservation<history::HistoryService,
+                          history::HistoryServiceObserver>
+      scoped_history_service_observer_{this};
+
   base::WeakPtrFactory<UsageStatsBridge> weak_ptr_factory_{this};
 
   DISALLOW_COPY_AND_ASSIGN(UsageStatsBridge);