DevTools: mark inspector/extensions/extensions-sidebar.html as failing for now

BUG=338752
TBR=dgozman

Review URL: https://codereview.chromium.org/136643003

git-svn-id: svn://svn.chromium.org/blink/trunk@165952 bbb929c8-8fbe-4397-9dbb-9b2b20218538