Separate and update PDF_DIFF_TRACE_IN_PNG

experimental/PdfViewer/inc/SkPdfDiffEncoder.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 SkPdfDiffEncoder_DEFINED
+#define SkPdfDiffEncoder_DEFINED
+
+struct PdfToken;
+
+namespace SkPdfDiffEncoder {
+    /**
+     * If PDF_TRACE_DIFF_IN_PNG is defined, the PDF commands so far are written
+     * to a file with the difference created by using this token highlighted.
+     * The file is named "/tmp/log_step_by_step/step-%i-%s.png", where %i is
+     * the number of the command and %s is the name of the command. If
+     * PDF_TRACE_DIFF_IN_PNG is not defined this function does nothing.
+     * TODO(scroggo): Pass SkPdfContext and SkCanvas for info.
+     */
+    void WriteToFile(PdfToken*);
+};
+
+#endif // SkPdfDiffEncoder_DEFINED