Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/objectivec/GPBRootObject.h

Index: third_party/protobuf/objectivec/GPBRootObject.h
diff --git a/third_party/protobuf/objectivec/GPBRootObject.h b/third_party/protobuf/objectivec/GPBRootObject.h
index d2e2aebfcc57ab092344f550fd8dcb535a4545e2..c05b5c620f37a45fb775c3f187fc9e525d9a2a45 100644
--- a/third_party/protobuf/objectivec/GPBRootObject.h
+++ b/third_party/protobuf/objectivec/GPBRootObject.h
@@ -34,17 +34,12 @@
 
 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