Design doc: Running DevTools end-to-end tests with host bindings and the real DevToolsWindow

I wrote a small design doc (https://docs.google.com/document/d/1CYh96kJSNtqGyyLJxYF3Omtqdg52o1FgE3aIKrHnHWU/edit?usp=sharing) on how we could improve the DevTools e2e suite to actually test what we ship and get some performance wins along the way.

Cheers, Simon