Fix back-forward cache in rustdoc frontend.
Fix back-forward cache in rustdoc frontend.

Rustdoc's frontend set a no-op unload handler, specifically to disable
Firefox's back-forward cache because it caused a bug. It's nice to
allow the back-forward cache because it permits faster navigations.

This change addresses the issues that were caused by back-forward cache.

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/1.5/Using_Firefox_1.5_caching
https://web.dev/bfcache/

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust