[Sync] Don't DCHECK if we can't find the window for a tab

If the model association happens at the right time (i.e. when a tab is being
dragged from one window to another), the tab strip model may be out of date.
Just handle this gracefully for now and return false.

BUG=282590
TBR=haitao@chromium.org

Review URL: https://codereview.chromium.org/81193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238338 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/sync/browser_synced_window_delegate.cc

@@ -47,7 +47,8 @@ bool BrowserSyncedWindowDelegate::IsTabPinned(
     if (tab == current)
       return browser_->tab_strip_model()->IsTabPinned(i);
   }
-  NOTREACHED();
+  // The window and tab are not always updated atomically, so it's possible
+  // one of the values was stale. We'll retry later, just ignore for now.
   return false;
 }