Fix copyImage for WebGL elements.

Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 423 matching lines @@
     m_actualAttributes.antialias = multisample();
     return true;
 }
 
 bool DrawingBuffer::copyToPlatformTexture(blink::WebGraphicsContext3D* context, Platform3DObject texture, GLenum internalFormat, GLenum destType, GLint level, bool premultiplyAlpha, bool flipY, bool fromFrontBuffer)
 {
     if (!m_context->makeContextCurrent())
         return false;
 
     GLint textureId = m_colorBuffer.textureId;
-    if (fromFrontBuffer)
+    if (fromFrontBuffer && m_frontColorBuffer.textureId)
         textureId = m_frontColorBuffer.textureId;
 
     if (m_contentsChanged) {
         if (m_multisampleMode != None) {
             commit();
             if (!m_framebufferBinding)
                 bind();
             else
                 restoreFramebufferBinding();
         }
@@ skipping 608 matching lines @@
 void DrawingBuffer::deleteChromiumImageForTexture(TextureInfo* info)
 {
     if (info->imageId) {
         m_context->releaseTexImage2DCHROMIUM(GL_TEXTURE_2D, info->imageId);
         m_context->destroyImageCHROMIUM(info->imageId);
         info->imageId = 0;
     }
 }
 
 } // namespace WebCore