Add 'emulateNetworkConditions' to protocol.json

This is the first part.
Final goal: https://codereview.chromium.org/249463002

BUG=245436

Review URL: https://codereview.chromium.org/176813008

git-svn-id: svn://svn.chromium.org/blink/trunk@172518 bbb929c8-8fbe-4397-9dbb-9b2b20218538