Generate correct error from WEBGL_draw_buffers extension.

third_party/WebKit/Source/modules/webgl/WebGLDrawBuffers.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 54 matching lines @@
 
 void WebGLDrawBuffers::drawBuffersWEBGL(const Vector<GLenum>& buffers)
 {
     WebGLExtensionScopedContext scoped(this);
     if (scoped.isLost())
         return;
     GLsizei n = buffers.size();
     const GLenum* bufs = buffers.data();
     if (!scoped.context()->m_framebufferBinding) {
         if (n != 1) {
-            scoped.context()->synthesizeGLError(GL_INVALID_VALUE, "drawBuffersWEBGL", "more than one buffer");
+            scoped.context()->synthesizeGLError(GL_INVALID_OPERATION, "drawBuffersWEBGL", "must provide exactly one buffer");
             return;
         }
         if (bufs[0] != GL_BACK && bufs[0] != GL_NONE) {
             scoped.context()->synthesizeGLError(GL_INVALID_OPERATION, "drawBuffersWEBGL", "BACK or NONE");
             return;
         }
         // Because the backbuffer is simulated on all current WebKit ports, we need to change BACK to COLOR_ATTACHMENT0.
         GLenum value = (bufs[0] == GL_BACK) ? GL_COLOR_ATTACHMENT0 : GL_NONE;
         scoped.context()->webContext()->drawBuffersEXT(1, &value);
         scoped.context()->setBackDrawBuffer(bufs[0]);
@@ skipping 87 matching lines @@
     if (supportsDepth)
         context->deleteTexture(depth);
     if (supportsDepthStencil)
         context->deleteTexture(depthStencil);
     for (size_t i = 0; i < colors.size(); ++i)
         context->deleteTexture(colors[i]);
     return ok;
 }
 
 } // namespace blink