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

ui/gl/gl_bindings.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 UI_GL_GL_BINDINGS_H_
 #define UI_GL_GL_BINDINGS_H_
 
 // Includes the platform independent and platform dependent GL headers.
 // Only include this in cc files. It pulls in system headers, including
 // the X11 headers on linux, which define all kinds of macros that are
@@ skipping 106 matching lines @@
 #define GL_COMMANDS_ISSUED_CHROMIUM                      0x84F2
 
 /* GL_CHROMIUM_get_error_query */
 #define GL_GET_ERROR_QUERY_CHROMIUM                      0x84F3
 
 /* GL_CHROMIUM_command_buffer_latency_query */
 #define GL_LATENCY_QUERY_CHROMIUM                        0x84F4
 
 /* GL_CHROMIUM_async_pixel_transfers */
 #define GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM      0x84F5
+#define GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM          0x84F6
 
 // GL_OES_texure_3D
 #define GL_SAMPLER_3D_OES                                0x8B5F
 
 // GL_OES_depth24
 #define GL_DEPTH_COMPONENT24_OES                         0x81A6
 
 // GL_OES_depth32
 #define GL_DEPTH_COMPONENT32_OES                         0x81A7
 
@@ skipping 178 matching lines @@
 GL_EXPORT extern DriverEGL g_driver_egl;
 
 #endif
 
 // Find an entry point to the mock GL implementation.
 void* GL_BINDING_CALL GetMockGLProcAddress(const char* name);
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_BINDINGS_H_