SkDocument/PDF: new API

has a pdf/a switch.
sets metadata in factory.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1916093002

Review URL: https://codereview.chromium.org/1916093002
diff --git a/tests/CanvasTest.cpp b/tests/CanvasTest.cpp
index 284e3cd..f22b7d0 100644
--- a/tests/CanvasTest.cpp
+++ b/tests/CanvasTest.cpp
@@ -570,7 +570,7 @@
                           const TestData& d,
                           CanvasTestStep* testStep) {
     SkDynamicMemoryWStream outStream;
-    SkAutoTUnref<SkDocument> doc(SkDocument::CreatePDF(&outStream));
+    sk_sp<SkDocument> doc(SkDocument::MakePDF(&outStream));
 #if SK_SUPPORT_PDF
     REPORTER_ASSERT(reporter, doc);
 #else