Bug 1581903 - Use the frame origin in LMP.onFormSubmit. r=sfoster

Writing a test would be tedious as it involves subframes, password generation and the doorhanger and this change is trivial so I didn't make one. This code will also get rewritten to use BrowsingContexts properly to improve sandboxing shortly.

Differential Revision: https://phabricator.services.mozilla.com/D46860

--HG--
extra : moz-landing-system : lando

toolkit/components/passwordmgr/LoginManagerParent.jsm

@@ -597,12 +597,7 @@ this.LoginManagerParent = {
       formActionOrigin,
     });
 
-    let { browsingContext } = browser;
-    let framePrincipalOrigin =
-      browsingContext.currentWindowGlobal.documentPrincipal.origin;
-    let generatedPW = this._generatedPasswordsByPrincipalOrigin.get(
-      framePrincipalOrigin
-    );
+    let generatedPW = this._generatedPasswordsByPrincipalOrigin.get(origin);
     let autoSavedStorageGUID = "";
     if (generatedPW && generatedPW.storageGUID) {
       autoSavedStorageGUID = generatedPW.storageGUID;