SkPDF: Fix links to be more valid PDF/A

src/pdf/SkPDFDevice.cpp

Index: src/pdf/SkPDFDevice.cpp
diff --git a/src/pdf/SkPDFDevice.cpp b/src/pdf/SkPDFDevice.cpp
index 38e6d1f021506390204125f84c106132559d9f2f..5c933ee7d570232b415be8186771fb070631ff80 100644
--- a/src/pdf/SkPDFDevice.cpp
+++ b/src/pdf/SkPDFDevice.cpp
@@ -827,6 +827,9 @@ void SkPDFDevice::drawPoints(const SkDraw& d,
 static sk_sp<SkPDFDict> create_link_annotation(const SkRect& translatedRect) {
     auto annotation = sk_make_sp<SkPDFDict>("Annot");
     annotation->insertName("Subtype", "Link");
+    #ifdef SK_PDF_GENERATE_PDFA
+        annotation->insertInt("F", 4);  // required by ISO 19005
+    #endif
 
     auto border = sk_make_sp<SkPDFArray>();
     border->reserve(3);