Have Document::topDocument() return a reference

Source/core/html/canvas/WebGLRenderingContextBase.cpp

 /*
  * Copyright (C) 2009 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 5316 matching lines @@
 
     LocalFrame* frame = canvas()->document().frame();
     if (!frame)
         return;
 
     Settings* settings = frame->settings();
 
     if (!frame->loader().client()->allowWebGL(settings && settings->webGLEnabled()))
         return;
 
-    blink::WebGraphicsContext3D::Attributes attributes = m_requestedAttributes->attributes(canvas()->document().topDocument()->url().string(), settings);
+    blink::WebGraphicsContext3D::Attributes attributes = m_requestedAttributes->attributes(canvas()->document().topDocument().url().string(), settings);
     OwnPtr<blink::WebGraphicsContext3D> context = adoptPtr(blink::Platform::current()->createOffscreenGraphicsContext3D(attributes));
     if (!context) {
         if (m_contextLostMode == RealLostContext) {
             m_restoreTimer.startOneShot(secondsBetweenRestoreAttempts);
         } else {
             // This likely shouldn't happen but is the best way to report it to the WebGL app.
             synthesizeGLError(GL_INVALID_OPERATION, "", "error restoring context");
         }
         return;
     }
@@ skipping 207 matching lines @@
         if (m_textureUnits[i].m_texture2DBinding
             || m_textureUnits[i].m_textureCubeMapBinding) {
             m_onePlusMaxNonDefaultTextureUnit = i + 1;
             return;
         }
     }
     m_onePlusMaxNonDefaultTextureUnit = 0;
 }
 
 } // namespace WebCore