refactor: prefer to inherit observer classes privately

[ckerr]

Description of Change

We have a lot of classes that inherit from upstream FooObserver classes to manage some state, e.g. AutofillDriverFactory which inherits from content::WebContentsObserver so that it can know when to close popups by overriding DidFinishNavigation().

But since WebContentsObserver is 900+ lines long, that public inheritance means AutofillDriverFactory exposes a lot of public API to its clients. At best, it's just excess surface area. At worst, clients might accidentally use that API and cause a bug.

This PR tries to inherit from upstream FooObservers privately where possible.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none.