Sheriff-O-Matic: Fix karma for testing

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180017

[ojan, 2014-08-11 23:33:19]

lgtm

[michaelpg, 2014-08-11 23:33:33]

The CQ bit was checked by michaelpg@chromium.org

[commit-bot: I haz the power, 2014-08-11 23:34:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/michaelpg@chromium.org/459223002/140001

[commit-bot: I haz the power, 2014-08-12 00:31:22]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/22146)

[commit-bot: I haz the power, 2014-08-12 01:09:31]

Change committed as 180017