Workaround DCHECK in SavedTabGroupModelListener

Temporary workaround to silence the DCHECK failure "Observers can only
be added once!" in SavedTabGroupModelListener::OnBrowserAdded().

(cherry picked from commit 24aa3fb)

Bug: 1345680, 1358609
Change-Id: Idfd1c0c1e5b89cc803edbaeaf470dc52665d5f15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3870846
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1042884}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3895476
Auto-Submit: David Pennington <dpenning@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/branch-heads/5195@{#1128}
Cr-Branched-From: 7aa3f07-refs/heads/main@{#1027018}

chrome/browser/ui/tabs/saved_tab_groups/saved_tab_group_model_listener.cc

@@ -49,6 +49,10 @@ TabStripModel* SavedTabGroupModelListener::GetTabStripModelWithTabGroupId(
 void SavedTabGroupModelListener::OnBrowserAdded(Browser* browser) {
   if (model_->profile() != browser->profile())
     return;
+  if (observed_browsers_.count(browser)) {
+    // TODO(crbug.com/1345680): Investigate the root cause of duplicate calls.
+    return;
+  }
   observed_browsers_.insert(browser);
   browser->tab_strip_model()->AddObserver(this);
 }