Correct iOS build for latest Xcode beta.

ios/public/test/test_updatable_resource_provider.mm

Index: ios/public/test/test_updatable_resource_provider.mm
diff --git a/ios/public/test/test_updatable_resource_provider.mm b/ios/public/test/test_updatable_resource_provider.mm
index de8459f2ec0c8636be5d7bd521b67f1c0ec1fa3a..32573a13245953537dad4617984e3734efcf7cf4 100644
--- a/ios/public/test/test_updatable_resource_provider.mm
+++ b/ios/public/test/test_updatable_resource_provider.mm
@@ -43,6 +43,7 @@
 - (instancetype)initWithDelegate:(id<UpdatableResourceDelegate>)delegate
                            plist:(NSString*)resource_identifier
     NS_DESIGNATED_INITIALIZER;
+- (instancetype)init NS_UNAVAILABLE;
 @end
 
 @implementation TestUpdatableResource {
@@ -50,6 +51,11 @@
   base::scoped_nsprotocol<id<UpdatableResourceDescriptorBridge>> _descriptor;
 }
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (instancetype)initWithDelegate:(id<UpdatableResourceDelegate>)delegate
                            plist:(NSString*)resourceIdentifier {
   if (self = [super init]) {