Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/objectivec/GPBRootObject.h

 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 Google Inc.  All rights reserved.
 // https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
@@ skipping 16 matching lines @@
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #import <Foundation/Foundation.h>
 
 @class GPBExtensionRegistry;
 
 NS_ASSUME_NONNULL_BEGIN
 
-/**
- * Every generated proto file defines a local "Root" class that exposes a
- * GPBExtensionRegistry for all the extensions defined by that file and
- * the files it depends on.
- **/
+/// Every generated proto file defines a local "Root" class that exposes a
+/// @c GPBExtensionRegistry for all the extensions defined by that file and
+/// the files it depends on.
 @interface GPBRootObject : NSObject
 
-/**
- * @return An extension registry for the given file and all the files it depends
- * on.
- **/
+/// An extension registry for the given file and all the files it depends on.
 + (GPBExtensionRegistry *)extensionRegistry;
 
 @end
 
 NS_ASSUME_NONNULL_END