Fix the base::ObserverList example usage comment.

The observer class should either inherit from base::CheckedObserver, or
use the `Unchecked` property when accessing the observers from the list.

The provided example is in a half-way state that doesn't work, so I
updated it to the recommended method (using CheckedObserver).

Change-Id: I3062e7d6829240c1ffefbe6ae5a9444e238b4529
Reviewed-on: https://chromium-review.googlesource.com/c/1491811
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Fran├žois Doray <fdoray@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636044}
1 file changed