DevTools: Unskip and fix network tests

BUG=233124
TBR=pfeldman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/23451022

git-svn-id: svn://svn.chromium.org/blink/trunk@172764 bbb929c8-8fbe-4397-9dbb-9b2b20218538