SkPDF: allow PDF module to be disabled in DM and SampleApp

src/doc/SkDocument_PDF_None.cpp

Index: src/doc/SkDocument_PDF_None.cpp
diff --git a/src/doc/SkDocument_PDF_None.cpp b/src/doc/SkDocument_PDF_None.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..4759acf2a0db6e789c8ec979269a2adab3d8efd7
--- /dev/null
+++ b/src/doc/SkDocument_PDF_None.cpp
@@ -0,0 +1,9 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#include "SkDocument.h"
+SkDocument* SkDocument::CreatePDF(SkWStream*, SkScalar) { return  NULL; }
+SkDocument* SkDocument::CreatePDF(const char path[], SkScalar) { return NULL; }