[ios] Change WebStateListObserverBridge to use weak reference.

ios/chrome/browser/tabs/tab_model.mm

Index: ios/chrome/browser/tabs/tab_model.mm
diff --git a/ios/chrome/browser/tabs/tab_model.mm b/ios/chrome/browser/tabs/tab_model.mm
index 4d76abffa82bd541c0a2b5a7bcfb76cdc2659865..e14b377526a92e671ec7ec4b6dffb164a29b784f 100644
--- a/ios/chrome/browser/tabs/tab_model.mm
+++ b/ios/chrome/browser/tabs/tab_model.mm
@@ -145,6 +145,11 @@ void CleanCertificatePolicyCache(
   // notification, translate and forward events, update metrics, ...).
   std::vector<std::unique_ptr<WebStateListObserver>> _webStateListObservers;
 
+  // Strong references to id<WebStateListObserving> wrapped by non-owning
+  // WebStateListObserverBridges.
+  base::scoped_nsobject<NSArray<id<WebStateListObserving>>>
+      _retainedWebStateListObservers;
+
   // The delegate for sync.
   std::unique_ptr<TabModelSyncedWindowDelegate> _syncedWindowDelegate;
 
@@ -208,6 +213,7 @@ void CleanCertificatePolicyCache(
   for (const auto& webStateListObserver : _webStateListObservers)
     _webStateList->RemoveObserver(webStateListObserver.get());
   _webStateListObservers.clear();
+  _retainedWebStateListObservers.reset();
 
   _clearPoliciesTaskTracker.TryCancelAll();
 
@@ -283,12 +289,20 @@ void CleanCertificatePolicyCache(
     DCHECK(service);
     _sessionService.reset([service retain]);
 
-    _webStateListObservers.push_back(base::MakeUnique<
-                                     WebStateListObserverBridge>([
-        [TabModelClosingWebStateObserver alloc]
-        initWithTabModel:self
-          restoreService:IOSChromeTabRestoreServiceFactory::GetForBrowserState(
-                             _browserState)]));
+    base::scoped_nsobject<NSMutableArray<id<WebStateListObserving>>>
+        retainedWebStateListObservers([[NSMutableArray alloc] init]);
+
+    base::scoped_nsobject<TabModelClosingWebStateObserver>

[rohitrao (ping after 24h), 2017/04/11 12:51:42]

Should we just have individual ivars for the three observers, rather than
putting them into |retainedWebStateListObservers|?

[sdefresne, 2017/04/12 11:19:47]

They are never (and should never) be accessed as individual observers, so I
don't think it is worth keeping direct pointers to them.

+        tabModelClosingWebStateObserver([[TabModelClosingWebStateObserver alloc]
+            initWithTabModel:self
+              restoreService:IOSChromeTabRestoreServiceFactory::
+                                 GetForBrowserState(_browserState)]);
+    [retainedWebStateListObservers addObject:tabModelClosingWebStateObserver];
+
+    _webStateListObservers.push_back(
+        base::MakeUnique<WebStateListObserverBridge>(
+            tabModelClosingWebStateObserver));
+
     _webStateListObservers.push_back(
         base::MakeUnique<SnapshotCacheWebStateListObserver>(
             [SnapshotCache sharedInstance]));
@@ -298,14 +312,21 @@ void CleanCertificatePolicyCache(
               _tabUsageRecorder.get()));
     }
     _webStateListObservers.push_back(base::MakeUnique<TabParentingObserver>());
+
+    base::scoped_nsobject<TabModelSelectedTabObserver>
+        tabModelSelectedTabObserver(
+            [[TabModelSelectedTabObserver alloc] initWithTabModel:self]);
+    [retainedWebStateListObservers addObject:tabModelSelectedTabObserver];
     _webStateListObservers.push_back(
         base::MakeUnique<WebStateListObserverBridge>(
-            [[TabModelSelectedTabObserver alloc] initWithTabModel:self]));
+            tabModelSelectedTabObserver));
+
+    base::scoped_nsobject<TabModelObserversBridge> tabModelObserversBridge(
+        [[TabModelObserversBridge alloc] initWithTabModel:self
+                                        tabModelObservers:_observers.get()]);
+    [retainedWebStateListObservers addObject:tabModelObserversBridge];
     _webStateListObservers.push_back(
-        base::MakeUnique<WebStateListObserverBridge>(
-            [[TabModelObserversBridge alloc]
-                 initWithTabModel:self
-                tabModelObservers:_observers.get()]));
+        base::MakeUnique<WebStateListObserverBridge>(tabModelObserversBridge));
 
     auto webStateListMetricsObserver =
         base::MakeUnique<WebStateListMetricsObserver>();
@@ -314,6 +335,7 @@ void CleanCertificatePolicyCache(
 
     for (const auto& webStateListObserver : _webStateListObservers)
       _webStateList->AddObserver(webStateListObserver.get());
+    _retainedWebStateListObservers.reset([retainedWebStateListObservers copy]);
 
     if (window) {
       DCHECK([_observers empty]);