[Merge] lp:~osomon/webbrowser-app/stop-using-deprecated-navigation-apis into lp:webbrowser-app/staging

[Andrew Hayzen]

Review: Approve

LGTM, both webbrowser and webapp-container enable/disable the forward/back buttons correctly and the buttons function correctly.
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https://code.launchpad.net/~osomon/webbrowser-app/stop-using-deprecated-navigation-apis/+merge/313550
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