Deflake new defer-back test

The test can fail in cases where BFCache is disabled and some race
condition makes it go in an infinite loop (back/forward/...) etc.

Fixing the flakiness by synchronously changing the URL to "done"
before performing the navigation from "initial" to "next", so that
going back doesn't re-ignire the "initial" state.

Bug: 469466893
Change-Id: I6cf64d57bddef61febff2fc3f810dffc05c96c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7532837
Commit-Queue: Noam Rosenthal <[email protected]>
Reviewed-by: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1576557}

Author: noamr

navigation-api/navigate-event/defer/tentative/defer-back.html

@@ -22,8 +22,12 @@
       });
       break;
     case "initial":
-      window.onload = () =>
+      window.onload = () => {
+        // Changing the URL synchronously to avoid an infinite loop when going back in case there is
+        // no BFCache.
+        history.replaceState(null, "", "?phase=done");
         navigation.navigate("?phase=next", { history: "push" });
+      }
       break;
     case "next":
       window.onload = () => {