Update clean testing environment page.

[Garbee]

https://developer.chrome.com/devtools/docs/clean-testing-environment

A lot of buzz lately on an issue for remembering the dock position in Incognito from some dock-to-bottom lovers. We should update this page to help clarify the advantages of using a separate profile vs incognito. As well as showcase the history scrubber extension to help make a normal profile more like incognito.