Perform glReadPixels with PBOs in the gpu, if PBOs are available.

gpu/command_buffer/service/gles2_cmd_validation_implementation_autogen.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.
 
 // This file is auto-generated from
 // gpu/command_buffer/build_gles2_cmd_buffer.py
 // DO NOT EDIT!
 
 #ifndef GPU_COMMAND_BUFFER_SERVICE_GLES2_CMD_VALIDATION_IMPLEMENTATION_AUTOGEN_H_  // NOLINT
 #define GPU_COMMAND_BUFFER_SERVICE_GLES2_CMD_VALIDATION_IMPLEMENTATION_AUTOGEN_H_  // NOLINT
@@ skipping 282 matching lines @@
 static GLenum valid_query_parameter_table[] = {
   GL_CURRENT_QUERY_EXT,
 };
 
 static GLenum valid_query_target_table[] = {
   GL_ANY_SAMPLES_PASSED_EXT,
   GL_ANY_SAMPLES_PASSED_CONSERVATIVE_EXT,
   GL_COMMANDS_ISSUED_CHROMIUM,
   GL_LATENCY_QUERY_CHROMIUM,
   GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM,
+  GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM,
 };
 
 static GLenum valid_read_pixel_format_table[] = {
   GL_ALPHA,
   GL_RGB,
   GL_RGBA,
 };
 
 static GLenum valid_read_pixel_type_table[] = {
   GL_UNSIGNED_BYTE,
@@ skipping 352 matching lines @@
           valid_vertex_attribute_table, arraysize(
               valid_vertex_attribute_table)),
       vertex_pointer(
           valid_vertex_pointer_table, arraysize(valid_vertex_pointer_table)),
       zero_only(
           valid_zero_only_table, arraysize(valid_zero_only_table)) {
 }
 
 #endif  // GPU_COMMAND_BUFFER_SERVICE_GLES2_CMD_VALIDATION_IMPLEMENTATION_AUTOGEN_H_  // NOLINT