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

To disable PDF:

    GYP_DEFINES='skia_pdf=0' bin/sync-and-gyp
    ninja -C out/Debug dm SampleApp

When disabled, SkDocument::CreatePDF() always returns NULL.

Review URL: https://codereview.chromium.org/1279123007
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 2d12bfd..486feed 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -1171,8 +1171,10 @@
 
 void SampleWindow::saveToPdf()
 {
+#if SK_SUPPORT_PDF
     fSaveToPdf = true;
     this->inval(NULL);
+#endif  // SK_SUPPORT_PDF
 }
 
 SkCanvas* SampleWindow::beforeChildren(SkCanvas* canvas) {