Re: [w3c/manifest] Remove the default value for start_url (#670)

I think the above comments by @marcoscaceres @dominickng and @pkotwicz represent sufficient consensus around the issue.

I will land the change as-is on Monday (unless there is an objection here). This will be a breaking change, of sorts. Specifically:

- Technically, this changes the start URL for all manifests missing a `start_url` member.
- User agents following the current spec will still technically be compliant with the new spec, under the clause in §7.3: "Please note that the start URL is not necessarily the value of the `start_url` member: the user or user agent could have changed it when the application was added to home-screen or otherwise bookmarked." They'll essentially be user agents that override the `start_url` member with custom logic, which is allowed by the spec.
- User agents that do update to the new behaviour are not required to migrate existing app shortcuts, per the above clause.
- Google Chrome currently does not allow installation if `start_url` is missing, so any site that wants to be installed should already have an explicit `start_url`. (This won't break any sites that previously worked, at least in Chrome.)
  - Having said that, Chrome will want to update its implementation to allow manifests without a `start_url` to be installable, under the new rules.
- Potential compat risk if Chrome updates to respect the new default but other browsers do not. Developers may rely on this behaviour in Chrome. User agents ideally would not distinguish between an explicit `"start_url": "."` and a missing `start_url`.

I have added a new (temporary) non-normative note to the spec in the latest commit:

"This default was changed in June 2018. Manifest authors might wish to avoid relying on the default behaviour (and explicitly specify this member), as user agents might still respect the previous default (the document URL), or ignore the manifest entirely if `start_url` is missing."

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