Update third_party/protobuf to protobuf-v3.0.0-beta-3

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 e2af5d974837a58ec4a2ba7d1d0d020e2ea16d92..c05b5c620f37a45fb775c3f187fc9e525d9a2a45 100644
--- a/third_party/protobuf/objectivec/GPBRootObject.h
+++ b/third_party/protobuf/objectivec/GPBRootObject.h
@@ -34,11 +34,12 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-// All Root Objects derive from GPBRootObject. It supplies a registry
-// for derived classes to register their extensions to.
+/// 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
 
-// Per class registry.
+/// An extension registry for the given file and all the files it depends on.
 + (GPBExtensionRegistry *)extensionRegistry;
 
 @end