Remove virtual clear in PdfViewer's SkNulCanvas

Causing compilation complaints on Windows

Review URL: https://codereview.chromium.org/783683004
diff --git a/experimental/PdfViewer/SkNulCanvas.h b/experimental/PdfViewer/SkNulCanvas.h
index 49d276d..c09e9eb 100644
--- a/experimental/PdfViewer/SkNulCanvas.h
+++ b/experimental/PdfViewer/SkNulCanvas.h
@@ -27,7 +27,6 @@
     virtual ~SkNulCanvas() {}
 
     virtual bool isDrawingToLayer() const SK_OVERRIDE {return false;}
-    virtual void clear(SkColor) SK_OVERRIDE {}
     virtual void drawPaint(const SkPaint& paint) SK_OVERRIDE {}
     virtual void drawPoints(PointMode mode, size_t count, const SkPoint pts[],
                             const SkPaint& paint) SK_OVERRIDE {}