Remove the SkCanvas comment API

experimental/PdfViewer/SkNulCanvas.h

Index: experimental/PdfViewer/SkNulCanvas.h
diff --git a/experimental/PdfViewer/SkNulCanvas.h b/experimental/PdfViewer/SkNulCanvas.h
index c8a95e9e9588631eb1859ac7cc182d1a5bdc1ead..9fbd506d41e924753a0fee61ddc852f64abf5144 100644
--- a/experimental/PdfViewer/SkNulCanvas.h
+++ b/experimental/PdfViewer/SkNulCanvas.h
@@ -26,9 +26,6 @@ public:
     explicit SkNulCanvas(const SkBitmap& bitmap) : SkCanvas(bitmap) {}
     virtual ~SkNulCanvas() {}
 
-    void beginCommentGroup(const char* description) override {}
-    void addComment(const char* kywd, const char* value) override {}
-    void endCommentGroup() override {}
     SkDrawFilter* setDrawFilter(SkDrawFilter* filter) override {return NULL;}
 
     bool isClipEmpty() const override { return false; }