Fix exception when running Sheriff-o-Matic tests.

Exception is caused by embedded-flakiness-dashboard. The iframe needs to
be added to the DOM to expose the contentWindow property.

By adding captureConsole: true to karma.conf.js, we will also see these
errors in the karma test runner in the future.

BUG=404415
R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/484753002

git-svn-id: svn://svn.chromium.org/blink/trunk@180485 bbb929c8-8fbe-4397-9dbb-9b2b20218538