Boost: Respect Save-Data header and tweak feature behavior

Some of our optimizations use extra bandwidth to improve performance (e.g. critical css, or in the future preloading links). We should respect the user's preferences and not cost them extra money. It's implemented in most browsers.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Save-Data

#### TODO
- [ ] Catalog the features which may cause extra bandwidth use (e.g.: Critical CSS)
- [ ] Explore the impact of turning these features off when the `Save-Data` header is in use.

The issue names no files, tests, or entry points. Start by cataloging optimizations that may increase bandwidth, then investigate how those features should behave when the Save-Data header is present; done means the affected features and their expected behavior are documented.

Written by the indexing model from the issue text.