Bug 1542347 - eliminate needless string flattening in OSReauthenticator; r=keeler

We already have a null-terminated `nsString` in this code; we don't need
to turn it into another null-terminated `nsString`.

Depends on D26355

Differential Revision: https://phabricator.services.mozilla.com/D26356

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

security/manager/ssl/OSReauthenticator.cpp

@@ -84,8 +84,7 @@ static nsresult ReauthenticateUserWindows(const nsACString& aPrompt,
   credui.cbSize = sizeof(credui);
   // TODO: maybe set parent (Firefox) here.
   credui.hwndParent = nullptr;
-  const nsString& tmpPrompt = NS_ConvertUTF8toUTF16(aPrompt);
-  const nsString& prompt = PromiseFlatString(tmpPrompt);
+  const nsString& prompt = NS_ConvertUTF8toUTF16(aPrompt);
   credui.pszMessageText = prompt.get();
   credui.pszCaptionText = nullptr;
   credui.hbmBanner = nullptr;  // ignored