[Mac] Fix for BrowserWindowTouchBar crash

Currently when BrowserTouchBarNotificationBridge is
destroyed, it will try to update the touch bar buttons,
which might have been destroyed.

This CL fixes the issue by having it just remove the
observer.

Bug: 852222
Change-Id: I8eae99985ea21c5b9f6ea5a3ffdf3998b80394e9
Reviewed-on: https://chromium-review.googlesource.com/1099483Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Sarah Chan <spqchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566970}

chrome/browser/ui/cocoa/browser_window_touch_bar.mm

@@ -151,7 +151,8 @@ class BrowserTouchBarNotificationBridge : public CommandObserver,
     if (model)
       model->RemoveObserver(this);
 
-    UpdateWebContents(nullptr);
+    if (contents_)
+      BookmarkTabHelper::FromWebContents(contents_)->RemoveObserver(this);
   }
 
   void UpdateTouchBar() { [owner_ invalidateTouchBar]; }