Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java

Index: third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java
diff --git a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java
index a22a74a03caeebacfaaca6705a87c53522274e62..0067392fa6058b48ea26e18d9d54323e12399596 100644
--- a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java
+++ b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/ExtensionRegistry.java
@@ -32,6 +32,7 @@ package com.google.protobuf;
 
 import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor;
+
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
@@ -100,7 +101,7 @@ public class ExtensionRegistry extends ExtensionRegistryLite {
 
   /** Get the unmodifiable singleton empty instance. */
   public static ExtensionRegistry getEmptyRegistry() {
-    return EMPTY_REGISTRY;
+    return EMPTY;
   }
 
 
@@ -242,11 +243,6 @@ public class ExtensionRegistry extends ExtensionRegistryLite {
     add(newExtensionInfo(extension), extension.getExtensionType());
   }
 
-  /** Add an extension from a generated file to the registry. */
-  public void add(final GeneratedMessage.GeneratedExtension<?, ?> extension) {
-    add((Extension<?, ?>) extension);
-  }
-
   static ExtensionInfo newExtensionInfo(final Extension<?, ?> extension) {
     if (extension.getDescriptor().getJavaType() ==
         FieldDescriptor.JavaType.MESSAGE) {
@@ -315,7 +311,7 @@ public class ExtensionRegistry extends ExtensionRegistryLite {
   private final Map<DescriptorIntPair, ExtensionInfo> mutableExtensionsByNumber;
 
   ExtensionRegistry(boolean empty) {
-    super(EMPTY_REGISTRY_LITE);
+    super(ExtensionRegistryLite.getEmptyRegistry());
     this.immutableExtensionsByName =
         Collections.<String, ExtensionInfo>emptyMap();
     this.mutableExtensionsByName =
@@ -325,7 +321,7 @@ public class ExtensionRegistry extends ExtensionRegistryLite {
     this.mutableExtensionsByNumber =
             Collections.<DescriptorIntPair, ExtensionInfo>emptyMap();
   }
-  static final ExtensionRegistry EMPTY_REGISTRY = new ExtensionRegistry(true);
+  private static final ExtensionRegistry EMPTY = new ExtensionRegistry(true);
 
   private void add(
       final ExtensionInfo extension,