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..a180f585a2606a59c68488ede5f6323fd7ffb37f 100644
--- a/ios/public/test/test_updatable_resource_provider.mm
+++ b/ios/public/test/test_updatable_resource_provider.mm
@@ -40,6 +40,7 @@
 // from the specified plist file.
 @interface TestUpdatableResource : NSObject<UpdatableResourceBridge>
 @property(nonatomic, readonly) id<UpdatableResourceDescriptorBridge> descriptor;
+- (instancetype)init NS_UNAVAILABLE;
 - (instancetype)initWithDelegate:(id<UpdatableResourceDelegate>)delegate
                            plist:(NSString*)resource_identifier
     NS_DESIGNATED_INITIALIZER;
@@ -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]) {