document.close from bool to void

Nothing meaningful is returned, it is inconsistent with the bulk of the rest of skia's api. The C api is waiting on this change as well.

Required chrome CL: https://codereview.chromium.org/2355343003/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2354403002

Review-Url: https://codereview.chromium.org/2354403002
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 3c2ee32..0565ee9 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -1263,9 +1263,7 @@
         }
         doc->endPage();
     }
-    if (!doc->close()) {
-        return "SkDocument::close() returned false";
-    }
+    doc->close();
     dst->flush();
     return "";
 }