Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

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

Index: third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java
diff --git a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java
index 4b0ba0fd6b7c564e1aec58f7ccd3832178758f15..2febaacecbae0bc2182bf8debbbcf66fc30afcc4 100644
--- a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java
+++ b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java
@@ -284,8 +284,29 @@ public class LazyFieldLite {
       return;
     }
 
-    // At this point we have two fully parsed messages.
-    setValue(this.value.toBuilder().mergeFrom(other.value).build());
+    // At this point we have two fully parsed messages. We can't merge directly from one to the
+    // other because only generated builder code contains methods to mergeFrom another parsed
+    // message. We have to serialize one instance and then merge the bytes into the other. This may
+    // drop extensions from one of the messages if one of the values had an extension set on it
+    // directly.
+    //
+    // To mitigate this we prefer serializing a message that has an extension registry, and
+    // therefore a chance that all extensions set on it are in that registry.
+    //
+    // NOTE: The check for other.extensionRegistry not being null must come first because at this
+    // point in time if other.extensionRegistry is not null then this.extensionRegistry will not be
+    // null either.
+    if (other.extensionRegistry != null) {
+      setValue(mergeValueAndBytes(this.value, other.toByteString(), other.extensionRegistry));
+      return;
+    } else if (this.extensionRegistry != null) {
+      setValue(mergeValueAndBytes(other.value, this.toByteString(), this.extensionRegistry));
+      return;
+    } else {
+      // All extensions from the other message will be dropped because we have no registry.
+      setValue(mergeValueAndBytes(this.value, other.toByteString(), EMPTY_REGISTRY));
+      return;
+    }
   }
   
   /**