Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/objectivec/GPBUnknownField.m

Index: third_party/protobuf/objectivec/GPBUnknownField.m
diff --git a/third_party/protobuf/objectivec/GPBUnknownField.m b/third_party/protobuf/objectivec/GPBUnknownField.m
index 30efe7563b712d61e6404b6597bd3dcfaffc64ed..0e29bde50f66b5861d93a180fa36a5b5e1b85c37 100644
--- a/third_party/protobuf/objectivec/GPBUnknownField.m
+++ b/third_party/protobuf/objectivec/GPBUnknownField.m
@@ -67,12 +67,6 @@
   [super dealloc];
 }
 
-// Direct access is use for speed, to avoid even internally declaring things
-// read/write, etc. The warning is enabled in the project to ensure code calling
-// protos can turn on -Wdirect-ivar-access without issues.
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdirect-ivar-access"
-
 - (id)copyWithZone:(NSZone *)zone {
   GPBUnknownField *result =
       [[GPBUnknownField allocWithZone:zone] initWithNumber:number_];
@@ -329,6 +323,4 @@
   }
 }
 
-#pragma clang diagnostic pop
-
 @end