BindTransformFeedback(0) is legal.

gpu/command_buffer/service/context_group.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GPU_COMMAND_BUFFER_SERVICE_CONTEXT_GROUP_H_
 #define GPU_COMMAND_BUFFER_SERVICE_CONTEXT_GROUP_H_
 
 #include <string>
 #include <vector>
 #include "base/basictypes.h"
@@ skipping 184 matching lines @@
   void RemoveSamplerId(GLuint client_id) {
     samplers_id_map_.erase(client_id);
   }
 
   void AddTransformFeedbackId(GLuint client_id, GLuint service_id) {
     transformfeedbacks_id_map_[client_id] = service_id;
   }
 
   bool GetTransformFeedbackServiceId(
       GLuint client_id, GLuint* service_id) const {
+    if (client_id == 0) {
+      // Default one.
+      if (service_id)
+        *service_id = 0;
+      return true;
+    }
     base::hash_map<GLuint, GLuint>::const_iterator iter =
         transformfeedbacks_id_map_.find(client_id);
     if (iter == transformfeedbacks_id_map_.end())
       return false;
     if (service_id)
       *service_id = iter->second;
     return true;
   }
 
   void RemoveTransformFeedbackId(GLuint client_id) {
@@ skipping 77 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(ContextGroup);
 };
 
 }  // namespace gles2
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_SERVICE_CONTEXT_GROUP_H_