WebExtensionToolbarFeature may not remove stale browser actions

[csadilek]

There's a case we're not handling properly. If an extension (add-on) is uninstalled, we remove all browser actions in the store. When switching back to the BrowserFragment, we should no longer see these actions in the toolbar. This currently only works if a new view is created and we get a new toolbar instance. However, it's possible for the view and toolbar to be reused (onCreateView would never be called) in which case stale browser actions would remain in the toolbar. This is because our current logic in WebExtensionToolbarFeature only adds actions to the toolbar (and updates them as needed), but it doesn't remove actions.

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