DevTools: gardening: mark a few inspector tests as flaky

BUG=420008
TBR= yurys@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/623083002

git-svn-id: svn://svn.chromium.org/blink/trunk@183189 bbb929c8-8fbe-4397-9dbb-9b2b20218538