Have Document::topDocument() return a reference

Source/core/html/canvas/WebGLRenderingContext.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 61 matching lines @@
         canvas->dispatchEvent(WebGLContextEvent::create(EventTypeNames::webglcontextcreationerror, false, true, "Web page was not allowed to create a WebGL context."));
         return nullptr;
     }
 
     // The only situation that attrs is null is through Document::getCSSCanvasContext().
     RefPtr<WebGLContextAttributes> defaultAttrs;
     if (!attrs) {
         defaultAttrs = WebGLContextAttributes::create();
         attrs = defaultAttrs.get();
     }
-    blink::WebGraphicsContext3D::Attributes attributes = attrs->attributes(document.topDocument()->url().string(), settings);
+    blink::WebGraphicsContext3D::Attributes attributes = attrs->attributes(document.topDocument().url().string(), settings);
     OwnPtr<blink::WebGraphicsContext3D> context = adoptPtr(blink::Platform::current()->createOffscreenGraphicsContext3D(attributes));
     if (!context || !context->makeContextCurrent()) {
         canvas->dispatchEvent(WebGLContextEvent::create(EventTypeNames::webglcontextcreationerror, false, true, "Could not create a WebGL context."));
         return nullptr;
     }
 
     Extensions3DUtil extensionsUtil(context.get());
     if (extensionsUtil.supportsExtension("GL_EXT_debug_marker"))
         context->pushGroupMarkerEXT("WebGLRenderingContext");
 
@@ skipping 44 matching lines @@
 
     // Register draft extensions.
     registerExtension<EXTFragDepth>(m_extFragDepth, DraftExtension);
 
     // Register privileged extensions.
     registerExtension<WebGLDebugRendererInfo>(m_webglDebugRendererInfo, WebGLDebugRendererInfoExtension);
     registerExtension<WebGLDebugShaders>(m_webglDebugShaders, PrivilegedExtension);
 }
 
 } // namespace WebCore