Add a mechanism for command buffer to conditionally allow ES3 enums.

gpu/command_buffer/service/gles2_cmd_validation_implementation_autogen.h

 // Copyright 2014 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.
 
 // This file is auto-generated from
 // gpu/command_buffer/build_gles2_cmd_buffer.py
 // It's formatted by clang-format using chromium coding style:
 //    clang-format -i -style=chromium filename
 // DO NOT EDIT!
 
@@ skipping 25 matching lines @@
 static const GLenum valid_buffer_parameter_table[] = {
     GL_BUFFER_SIZE,
     GL_BUFFER_USAGE,
 };
 
 static const GLenum valid_buffer_target_table[] = {
     GL_ARRAY_BUFFER,
     GL_ELEMENT_ARRAY_BUFFER,
 };
 
+static const GLenum valid_buffer_target_table_es3[] = {
+    GL_COPY_READ_BUFFER,
+    GL_COPY_WRITE_BUFFER,
+    GL_PIXEL_PACK_BUFFER,
+    GL_PIXEL_UNPACK_BUFFER,
+    GL_TRANSFORM_FEEDBACK_BUFFER,
+    GL_UNIFORM_BUFFER,
+};
+
 static const GLenum valid_buffer_usage_table[] = {
     GL_STREAM_DRAW,
     GL_STATIC_DRAW,
     GL_DYNAMIC_DRAW,
 };
 
 static const GLenum valid_bufferfi_table[] = {
     GL_DEPTH_STENCIL,
 };
 
@@ skipping 688 matching lines @@
       vertex_attrib_size(valid_vertex_attrib_size_table,
                          arraysize(valid_vertex_attrib_size_table)),
       vertex_attrib_type(valid_vertex_attrib_type_table,
                          arraysize(valid_vertex_attrib_type_table)),
       vertex_attribute(valid_vertex_attribute_table,
                        arraysize(valid_vertex_attribute_table)),
       vertex_pointer(valid_vertex_pointer_table,
                      arraysize(valid_vertex_pointer_table)) {
 }
 
+void Validators::AddES3Values() {
+  buffer_target.AddValues(valid_buffer_target_table_es3,
+                          arraysize(valid_buffer_target_table_es3));
+}
+
 #endif  // GPU_COMMAND_BUFFER_SERVICE_GLES2_CMD_VALIDATION_IMPLEMENTATION_AUTOGEN_H_