Do not call glDrawBuffersARB when the extension is absent

gpu/command_buffer/service/gles2_cmd_decoder.cc

 // 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.
 
 #include "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <stdio.h>
 
 #include <algorithm>
 #include <list>
@@ skipping 3104 matching lines @@
       state_.SetDeviceColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
       glClearStencil(0);
       state_.SetDeviceStencilMaskSeparate(GL_FRONT, kDefaultStencilMask);
       state_.SetDeviceStencilMaskSeparate(GL_BACK, kDefaultStencilMask);
       glClearDepth(1.0f);
       state_.SetDeviceDepthMask(GL_TRUE);
       state_.SetDeviceCapabilityState(GL_SCISSOR_TEST, false);
       bool reset_draw_buffer = false;
       if ((backbuffer_needs_clear_bits_ | GL_COLOR_BUFFER_BIT) != 0 &&
           group_->draw_buffer() == GL_NONE) {
+        DCHECK(gfx::g_driver_gl.ext.b_GL_ARB_draw_buffers);
         reset_draw_buffer = true;
         GLenum buf = GL_BACK;
         if (GetBackbufferServiceId() != 0)  // emulated backbuffer
           buf = GL_COLOR_ATTACHMENT0;
         glDrawBuffersARB(1, &buf);
       }
       glClear(backbuffer_needs_clear_bits_);
       if (reset_draw_buffer) {
         GLenum buf = GL_NONE;
         glDrawBuffersARB(1, &buf);
@@ skipping 7781 matching lines @@
   }
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu