Bug 1860915, part 1 - Implement part of Document Policy for force-load-at-top. r=necko-reviewers,smaug,kershaw

This adds support for a piece of the Document Policy header parsing we need for
force-load-at-top, and sets a bit on Document if text fragments are enabled,
but this patch does not actually check the bit. That is done in the next
patch.

Differential Revision: https://phabricator.services.mozilla.com/D191798

netwerk/base/nsNetUtil.cpp

@@ -2699,6 +2699,46 @@ NS_GetCrossOriginEmbedderPolicyFromHeader(
   return nsILoadInfo::EMBEDDER_POLICY_NULL;
 }
 
+bool NS_GetForceLoadAtTopFromHeader(const nsACString& aHeader) {
+  nsCOMPtr<nsISFVService> sfv = mozilla::net::GetSFVService();
+
+  nsCOMPtr<nsISFVDictionary> dict;
+  if (NS_FAILED(sfv->ParseDictionary(aHeader, getter_AddRefs(dict)))) {
+    return false;
+  }
+  nsCOMPtr<nsISFVItemOrInnerList> iil;
+  if (NS_FAILED(dict->Get("force-load-at-top"_ns, getter_AddRefs(iil)))) {
+    return false;
+  }
+
+  nsCOMPtr<nsISFVItem> item(do_QueryInterface(iil));
+  if (!item) {
+    return false;
+  }
+
+  nsCOMPtr<nsISFVBareItem> bareItem;
+  if (NS_FAILED(item->GetValue(getter_AddRefs(bareItem)))) {
+    return false;
+  }
+
+  int32_t type;
+  if (NS_FAILED(bareItem->GetType(&type))) {
+    return false;
+  }
+
+  nsCOMPtr<nsISFVBool> boolItem(do_QueryInterface(bareItem));
+  if (!boolItem) {
+    return false;
+  }
+
+  bool b;
+  if (NS_FAILED(boolItem->GetValue(&b))) {
+    return false;
+  }
+
+  return b;
+}
+
 /** Given the first (disposition) token from a Content-Disposition header,
  * tell whether it indicates the content is inline or attachment
  * @param aDispToken the disposition token from the content-disposition header