Missing Python bindings for Chrome network emulation

[tomaszn]

Analogically to JS bindings prepared in #3624 and committed in d7bdd7c, Python bindings are missing the Chrome debugging protocol functionality for getting and setting network emulation settings.
This functionality was added to ChromeDriver in March of 2015 (https://chromium.googlesource.com/chromium/src.git/+/12a55ff7369b35adb29cd28f5e3dd4ef923e3e6c).

[p0deje]

I'm closing this issue - keep track of #4011.