Gardening: Suppress flaky test

New flaky test added in r156227 with release builds:
http/tests/inspector/network/network-document-initiator.html

BUG=272725
TBR=eustas, dominicc

Review URL: https://codereview.chromium.org/22889007

git-svn-id: svn://svn.chromium.org/blink/trunk@156244 bbb929c8-8fbe-4397-9dbb-9b2b20218538