Re: [manifest] Allow developers to detect when the user has added the site to their home screen (or equivalent) (#417)

@marcoscaceres [Chrome Platform Status][1] suggests [explainer.md][2] is the documentation for `beforeinstallprompt`. [LayoutTests][3] should tell what is implemented (assuming test coverage is good, @mounirlamouri?).

Without a strong use case I'd punt `e.platforms` for later. It exposes the `related_applications` that "are provided as options in an install prompt".

[1]: https://www.chromestatus.com/feature/6560913322672128
[2]: https://github.com/slightlyoff/AppInstallImprovements/blob/master/explainer.md#controlling-installation
[3]: https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/LayoutTests/app_banner/

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