Set MaxDrawBuffers to 1 if WEBGL_draw_buffers is not enabled in WebGL.

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 2465 matching lines @@
   GetShaderPrecisionFormatImpl(GL_FRAGMENT_SHADER, GL_HIGH_FLOAT,
                                range, &precision);
   resources.FragmentPrecisionHigh =
       PrecisionMeetsSpecForHighpFloat(range[0], range[1], precision);
 #endif
 
   if (force_webgl_glsl_validation_) {
     resources.OES_standard_derivatives = derivatives_explicitly_enabled_;
     resources.EXT_frag_depth = frag_depth_explicitly_enabled_;
     resources.EXT_draw_buffers = draw_buffers_explicitly_enabled_;
+    if (!draw_buffers_explicitly_enabled_)
+      resources.MaxDrawBuffers = 1;
   } else {
     resources.OES_standard_derivatives =
         features().oes_standard_derivatives ? 1 : 0;
     resources.ARB_texture_rectangle =
         features().arb_texture_rectangle ? 1 : 0;
     resources.OES_EGL_image_external =
         features().oes_egl_image_external ? 1 : 0;
     resources.EXT_draw_buffers =
         features().ext_draw_buffers ? 1 : 0;
     resources.EXT_frag_depth =
@@ skipping 7768 matching lines @@
   return error::kNoError;
 }
 
 // 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