WebGL: Fix assertion hit of gpu tests.

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 173 matching lines @@
     m_layerComposited = true;
 }
 
 blink::WebGraphicsContext3D* DrawingBuffer::context()
 {
     return m_context.get();
 }
 
 bool DrawingBuffer::prepareMailbox(blink::WebExternalTextureMailbox* outMailbox, blink::WebExternalBitmap* bitmap)
 {
-    // prepareMailbox() is always called after layout.
-    ASSERT(!m_destructionInProgress);
-
     if (!m_contentsChanged)
         return false;
 
+    if (m_destructionInProgress) {
+        // It can be hit in the following sequence.
+        // 1. WebGL draws something.
+        // 2. The compositor begins the frame.
+        // 3. Javascript makes a context lost using WEBGL_lose_context extension.
+        // 4. Here.
+        return false;
+    }
+
     m_context->makeContextCurrent();
 
     // Resolve the multisampled buffer into m_colorBuffer texture.
     if (m_multisampleMode != None)
         commit();
 
     if (bitmap) {
         bitmap->setSize(size());
 
         unsigned char* pixels = bitmap->pixels();
@@ skipping 796 matching lines @@
     }
 }
 
 void DrawingBuffer::texImage2DResourceSafe(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, GLint unpackAlignment)
 {
     ASSERT(unpackAlignment == 1 || unpackAlignment == 2 || unpackAlignment == 4 || unpackAlignment == 8);
     m_context->texImage2D(target, level, internalformat, width, height, border, format, type, 0);
 }
 
 } // namespace WebCore