Correct iOS build for latest Xcode beta.

ios/web/crw_browsing_data_store_unittest.mm

Index: ios/web/crw_browsing_data_store_unittest.mm
diff --git a/ios/web/crw_browsing_data_store_unittest.mm b/ios/web/crw_browsing_data_store_unittest.mm
index a35c7f954d88a6cbe702ed44aafc3cf61dad8cfb..84da93d1735d37bc45bb10485bc54ee7068caeaf 100644
--- a/ios/web/crw_browsing_data_store_unittest.mm
+++ b/ios/web/crw_browsing_data_store_unittest.mm
@@ -19,6 +19,7 @@
 // An observer to observe the |mode| key changes to a CRWBrowsingDataStore.
 // Used for testing purposes.
 @interface CRWTestBrowsingDataStoreObserver : NSObject
+- (instancetype)init UNAVAILABLE_ATTRIBUTE;
 // Designated init. |browsingDataStore| cannot be null.
 - (instancetype)initWithBrowsingDataStore:
         (CRWBrowsingDataStore*)browsingDataStore NS_DESIGNATED_INITIALIZER;
@@ -34,6 +35,11 @@
 
 @synthesize modeChangeCount = _modeChangeCount;
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (instancetype)initWithBrowsingDataStore:
         (CRWBrowsingDataStore*)browsingDataStore {
   self = [super init];