Sheriff-O-Matic: Fix karma for testing

Including polymer.html in karma.conf.js makes platform.js throw
"Possible attempt to load Polymer twice" when a polymer component is imported.

I think this is because polymer.html is loaded twice: once by karma and again on
the first time that the browser encounters a <link href="polymer.html">. I believe
this might happen because of the way karma includes files by appending some kind
of hash to them.

If polymer.html is parsed twice, then the <script src="polymer.js"> also runs twice.

This CL adds karma-loader.html which imports polymer.html (so it should avoid
importing it a second time) and adds a listener for polymer-ready, which I didn't
think was necessary but seems to resolve some flakiness in karma.

BUG=400387
R=ojan@chromium.org,esprehn@chromium.org

Review URL: https://codereview.chromium.org/459223002

git-svn-id: svn://svn.chromium.org/blink/trunk@180017 bbb929c8-8fbe-4397-9dbb-9b2b20218538