UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0

Steps to reproduce the problem:
We use the remote debugging protocol to do performance testing of our pages. Generally we want each page load to happen with a clean cache with only explicitly set cookies. For some pages we want to see how they perform after navigating from a given start page. It would be nice if we could create a browser context (A), navigate to the start page, then when we want to test our target page we create a new browser context (B) which is a fork from A. Any subsequent changes in B (new cache entries, changes in cookies, etc) only affect B and do not modify the state of A.

What is the expected behavior?

What went wrong?

Did this work before? N/A 

Chrome version: 63.0.3239.84  Channel: n/a
OS Version: 10.0
Flash Version: