Down-integrate internal changes to github. (#5566)

* Down-integrate internal changes to github.

* fix csharp conformance test

* add comments in conformance.proto for text format

* fix comments

* fix comments, re-generated csharp file

* fix comments, re-generated csharp file
diff --git a/conformance/conformance_test.cc b/conformance/conformance_test.cc
index 265a053..b743e8e 100644
--- a/conformance/conformance_test.cc
+++ b/conformance/conformance_test.cc
@@ -85,6 +85,11 @@
       break;
     }
 
+    case conformance::TEXT_FORMAT: {
+      request_.set_text_payload(input);
+      break;
+    }
+
     default:
       GOOGLE_LOG(FATAL) << "Unspecified input format";
   }
@@ -131,6 +136,8 @@
       return "ProtobufInput";
     case conformance::JSON:
       return "JsonInput";
+    case conformance::TEXT_FORMAT:
+      return "TextFormatInput";
     default:
       GOOGLE_LOG(FATAL) << "Unspecified output format";
   }
@@ -144,6 +151,8 @@
       return "ProtobufOutput";
     case conformance::JSON:
       return "JsonOutput";
+    case conformance::TEXT_FORMAT:
+      return "TextFormatOutput";
     default:
       GOOGLE_LOG(FATAL) << "Unspecified output format";
   }
@@ -341,6 +350,8 @@
       return "JSON";
     case conformance::JSPB:
       return "JSPB";
+    case conformance::TEXT_FORMAT:
+      return "TEXT_FORMAT";
     case conformance::UNSPECIFIED:
       return "UNSPECIFIED";
     default: