Disable prefetching on SavePasswords

When prefetching is enabled, the collection view can get configured cells
in advance of them becoming visible. If the item updates after a cell was
prefetched, but before it is visible, the cell cannot be reconfigured.

For SavePasswords, the Export button may need to be enabled/disabled after
being prefetched, so prefetching needs to be disabled.

Bug: 812171
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ieef7b9cc3b91e237638277654e5d373c3906fb47
Reviewed-on: https://chromium-review.googlesource.com/924307Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537441}

ios/chrome/browser/ui/settings/save_passwords_collection_view_controller.mm

@@ -227,6 +227,11 @@ void SavePasswordsConsumer::OnGetPasswordStoreResults(
   [passwordManagerEnabled_ setObserver:nil];
 }
 
+- (void)viewDidLoad {
+  [super viewDidLoad];
+  self.collectionView.prefetchingEnabled = NO;
+}
+
 #pragma mark - SettingsRootCollectionViewController
 
 - (void)loadModel {
@@ -497,14 +502,13 @@ void SavePasswordsConsumer::OnGetPasswordStoreResults(
       self.passwordExporter.exportState == ExportState::IDLE) {
     exportPasswordsItem_.textColor = [[MDCPalette greyPalette] tint900];
     exportPasswordsItem_.accessibilityTraits = UIAccessibilityTraitButton;
-    [self reconfigureCellsForItems:@[ exportPasswordsItem_ ]];
     exportEnabled_ = YES;
   } else {
     exportPasswordsItem_.textColor = [[MDCPalette greyPalette] tint500];
     exportPasswordsItem_.accessibilityTraits = UIAccessibilityTraitNotEnabled;
-    [self reconfigureCellsForItems:@[ exportPasswordsItem_ ]];
     exportEnabled_ = NO;
   }
+  [self reconfigureCellsForItems:@[ exportPasswordsItem_ ]];
 }
 
 - (void)startPasswordsExportFlow {