Added classname for silent token renewal iframe (#6985)

Porting change from v2 PR:
#6960

Author: lalimasharda

change/@azure-msal-browser-e1682ade-b51d-46db-bd8d-9aaf52b0ba29.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Add classname to silent token renewal iframe HTML element #6985",
+  "packageName": "@azure/msal-browser",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

lib/msal-browser/src/interaction_handler/SilentHandler.ts

@@ -195,6 +195,7 @@ function loadFrameSync(urlNavigate: string): HTMLIFrameElement {
 function createHiddenIframe(): HTMLIFrameElement {
     const authFrame = document.createElement("iframe");
 
+    authFrame.className = "msalSilentIframe";
     authFrame.style.visibility = "hidden";
     authFrame.style.position = "absolute";
     authFrame.style.width = authFrame.style.height = "0";