Add embedded flakiness dashboard to sheriff-o-matic

Create a new Polymer widget for the embedded flakiness
dashboard.

Sadly the height isn't always right, as the underlying
flakiness dashboard code doesn't always post us a message
with the proper height. This issue existed in the previous
version as well.

NOTRY=true

Review URL: https://codereview.chromium.org/399473003

git-svn-id: svn://svn.chromium.org/blink/trunk@178222 bbb929c8-8fbe-4397-9dbb-9b2b20218538