SkPDF: AutoTUnref<T> changes in use

    ::detach()      -> ::release()
    ::operator T*() -> ::get()

This makes all use of AutoTUnref work the same as sk_sp.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1772493002

Review URL: https://codereview.chromium.org/1772493002
diff --git a/src/pdf/SkPDFMetadata.cpp b/src/pdf/SkPDFMetadata.cpp
index 51619c6..8359786 100644
--- a/src/pdf/SkPDFMetadata.cpp
+++ b/src/pdf/SkPDFMetadata.cpp
@@ -45,7 +45,7 @@
     if (fModified) {
         dict->insertString("ModDate", pdf_date(*fModified.get()));
     }
-    return dict.detach();
+    return dict.release();
 }
 
 #ifdef SK_PDF_GENERATE_PDFA
@@ -92,7 +92,7 @@
             SkString(reinterpret_cast<const char*>(&doc), sizeof(UUID)));
     array->appendString(
             SkString(reinterpret_cast<const char*>(&instance), sizeof(UUID)));
-    return array.detach();
+    return array.release();
 }
 
 // Improvement on SkStringPrintf to allow for arbitrarily long output.