refactor: prefer to inherit observer classes privately

[ckerr]

Description of Change

We have a lot of classes that inherit from upstream observer classes to manage internal state, e.g. our AutofillDriverFactory inherits from content::WebContentsObserver so that it can know when to close popups.

Since WebContentsObserver inherits AutofillDriverFactory publicly, we are accidentally exposing a lot of public API (it's a 900+ LOC class!) to WebContentsObserver's clients. We shouldn't do that; it might accidentally get used.

This PR uses private inheritance instead..

Checklist

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

Release Notes

Notes: none.