DevTools: cleanup Network.emulateNetworkConditions

WARNING: should be landed after corresponding chromium patch.

BUG=245436

Review URL: https://codereview.chromium.org/342433002

git-svn-id: svn://svn.chromium.org/blink/trunk@176504 bbb929c8-8fbe-4397-9dbb-9b2b20218538