nsIWebBrowserChrome::CHROME_PRIVATE_WINDOW should be replaced with OriginAttributes.mPrivateBrowsing flag.
All of CHROME_PRIVATE_WINDOW handling in child process is broken, and we just happen to propagate private browsing flag properly since we end up calling ApplyShowInfo. Took quite some time for baku and me to debug the setup for Bug 1280105 to ensure e10s-FF48 doesn't have totally broken pb in window.open case. Luckily Bug 1280105 removes the flag handling in child process, but I think there is still more very error prone cases in parent.
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