SkPDF: Add SkPDFCanvas to intercept some draw calls

src/pdf/SkPDFCanvas.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkPDFCanvas_DEFINED
+#define SkPDFCanvas_DEFINED
+
+#include "SkCanvas.h"
+
+class SkPDFDevice;
+
+class SkPDFCanvas : public SkCanvas {
+public:
+    SkPDFCanvas(const sk_sp<SkPDFDevice>&);
+    ~SkPDFCanvas();
+
+protected:
+    void onDrawBitmapNine(const SkBitmap&, const SkIRect&, const SkRect&,
+                          const SkPaint*) override;
+
+    void onDrawImageNine(const SkImage*, const SkIRect&, const SkRect&,
+                         const SkPaint*) override;
+
+    void onDrawImageRect(const SkImage*,
+                         const SkRect*,
+                         const SkRect&,
+                         const SkPaint*,
+                         SkCanvas::SrcRectConstraint) override;
+
+    void onDrawBitmapRect(const SkBitmap&,
+                          const SkRect*,
+                          const SkRect&,
+                          const SkPaint*,
+                          SkCanvas::SrcRectConstraint) override;
+};
+
+#endif  // SkPDFCanvas_DEFINED