pdfviewer: (more code cleanup): class documentation and comments

experimental/PdfViewer/SkNulCanvas.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkNulCanvas_DEFINED
 #define SkNulCanvas_DEFINED
 
 #include "SkCanvas.h"
 
+/** \class SkNulCanvas
+ *
+ *   Nul Canvas is a canvas that does nothing. It is used to measure the perf of just parsing
+ *   a pdf, without actually rendering anything.
+ *
+ */
 class SK_API SkNulCanvas : public SkCanvas {
 public:
     SK_DECLARE_INST_COUNT(SkNulCanvas);
 
     SkNulCanvas() {}
     explicit SkNulCanvas(SkBaseDevice* device) : SkCanvas(device) {}
 
     explicit SkNulCanvas(const SkBitmap& bitmap) : SkCanvas(bitmap) {}
     virtual ~SkNulCanvas() {}
 
@@ skipping 67 matching lines @@
 
 protected:
     virtual SkCanvas* canvasForDrawIter() {return NULL;}
     virtual SkBaseDevice* setDevice(SkBaseDevice* device) {return NULL;}
 
 private:
     typedef SkCanvas INHERITED;
 };
 
 #endif  // SkNulCanvas_DEFINED