Add mocha to third_party

Mocha is a test framework that makes asynchronous tests simple
and helps batch multiple logical tests into a single browser test.
This makes writing a web component test framework much, much easier.

See http://crrev.com/1124873002 for an example of how mocha could be
used to write a simple in-browser test of a custom element.

BUG=492889

Review URL: https://codereview.chromium.org/1140003003

Cr-Commit-Position: refs/heads/master@{#331880}
6 files changed