Correct iOS build for latest Xcode beta.

ios/web/crw_browsing_data_store.mm

Index: ios/web/crw_browsing_data_store.mm
diff --git a/ios/web/crw_browsing_data_store.mm b/ios/web/crw_browsing_data_store.mm
index 1d9fbe0d65dc91cf00977202ab5fbbf5e522831b..d78db6a55e4fa2c56035dc0a7dfd263269ab2263 100644
--- a/ios/web/crw_browsing_data_store.mm
+++ b/ios/web/crw_browsing_data_store.mm
@@ -45,6 +45,7 @@ NSString* const kStashOperationName = @"CRWBrowsingDataStore.STASH";
 // run. All remove operations need to be run on the same queue hence it is
 // shared with all CRWBrowsingDataStores.
 + (NSOperationQueue*)operationQueueForRemoveOperations;
+- (instancetype)init NS_UNAVAILABLE;
 
 // The array of all browsing data managers. Must be accessed from the main
 // thread.
@@ -176,6 +177,11 @@ NSString* const kStashOperationName = @"CRWBrowsingDataStore.STASH";
   return self;
 }
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (NSString*)description {
   NSString* format = @"<%@: %p; hasPendingOperations = { %@ }>";
   NSString* hasPendingOperationsString =