UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13) AppleWebKit/604.1.38 (KHTML, like Gecko) Version/11.0 Safari/604.1.38

Steps to reproduce the problem:
Currently the only way to set headless proxy settings is via the --proxy-server command line argument or with the C++ API `HeadlessBrowserContext::Builder::SetProxyConfig(...)`.  The latter is the only approach available if the developer wishes to use distinct (non-global) proxy settings for multiple browser contexts.

It would be great if this could instead be done through the DevTools Protocol, so that there is no need to create a custom application binary using the C++ API for this functionality alone.

A straightforward extension of the DevTools Protocol could be to add a new optional argument to `Target.createBrowserContext` called `proxyServer`.

This new argument could be a string matching the format accepted by `net::ProxyConfig::ParseFromString(...)` which is also the same format given by the global --proxy-server command line argument.

What is the expected behavior?

What went wrong?
We currently have no way to set the headless browser context proxy settings via the DevTools Protocol.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 61.0.3163.100  Channel: n/a
OS Version: OS X 10.13
Flash Version: