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/tests/CanvasTest.cpp b/tests/CanvasTest.cpp
index 970a370..78253ac 100644
--- a/tests/CanvasTest.cpp
+++ b/tests/CanvasTest.cpp
@@ -568,6 +568,8 @@
                           CanvasTestStep* testStep) {
     SkDynamicMemoryWStream outStream;
     SkAutoTUnref<SkDocument> doc(SkDocument::CreatePDF(&outStream));
+#if SK_SUPPORT_PDF
+    REPORTER_ASSERT(reporter, doc);
     SkCanvas* canvas = doc->beginPage(SkIntToScalar(d.fWidth),
                                       SkIntToScalar(d.fHeight));
     REPORTER_ASSERT(reporter, canvas);
@@ -575,6 +577,9 @@
     testStep->draw(canvas, d, reporter);
 
     REPORTER_ASSERT(reporter, doc->close());
+#else
+    REPORTER_ASSERT(reporter, !doc);
+#endif  // SK_SUPPORT_PDF
 }
 
 // The following class groups static functions that need to access