Bug 1635094 - Cleanup the ReferrerInfo code. r=ckerschb

The cast in InitWithNode is wrong. AsElement() asserts instead of
checking the flag, so we always pass an element (and if we didn't we'd
have type confusion problems). I audited the callers and we're fine.

Anyhow, always require an element, and add two convenience constructors
for C++ code.

Differential Revision: https://phabricator.services.mozilla.com/D73636

docshell/base/nsDocShell.cpp

@@ -11996,7 +11996,6 @@ nsresult nsDocShell::OnLinkClickSync(
   // referrer, since the current URI in this docshell may be a
   // new document that we're in the process of loading.
   RefPtr<Document> referrerDoc = aContent->OwnerDoc();
-  NS_ENSURE_TRUE(referrerDoc, NS_ERROR_UNEXPECTED);
 
   // Now check that the referrerDoc's inner window is the current inner
   // window for mScriptGlobal.  If it's not, then we don't want to
@@ -12029,12 +12028,9 @@ nsresult nsDocShell::OnLinkClickSync(
   GetIsExecutingOnLoadHandler(&inOnLoadHandler);
   uint32_t loadType = inOnLoadHandler ? LOAD_NORMAL_REPLACE : LOAD_LINK;
 
-  nsCOMPtr<nsIReferrerInfo> referrerInfo = new ReferrerInfo();
-  if (isElementAnchorOrArea) {
-    referrerInfo->InitWithNode(aContent);
-  } else {
-    referrerInfo->InitWithDocument(referrerDoc);
-  }
+  nsCOMPtr<nsIReferrerInfo> referrerInfo =
+      isElementAnchorOrArea ? new ReferrerInfo(*aContent->AsElement())
+                            : new ReferrerInfo(*referrerDoc);
 
   RefPtr<nsDocShellLoadState> loadState = new nsDocShellLoadState(aURI);
   loadState->SetReferrerInfo(referrerInfo);