Looks like 63 introduced a bug where the Chrome remote debug protocol Network.setExtraHTTPHeaders is no longer adding the headers to the initial navigation request.  My guess is that it is because of plznavigate and the navigation request being initiated from the browser and not the renderer.

For example, this webpagetest test: https://www.webpagetest.org/result/171207_1H_647bfdafac28e369757b0b6837595fe1/

adds a custom header "x-Bob: CustomHeaderHere" by sending the message:

{"params":{"headers":{"x-Bob":"CustomHeaderHere"}},"id":9,"method":"Network.setExtraHTTPHeaders"}

Looking through the individual requests the header is applied to all of the requests except for the first navigation request.

Remote debug protocol spec:

https://chromedevtools.github.io/devtools-protocol/tot/Network/#method-setExtraHTTPHeaders