[Gin] Add a basic unit testing framework

Previously, we were using JavaScript bindings to gtest to unit test JavaScript
code in Gin and Mojo. The gtest bindings were very basic and not very
idiomatic.

This CL introduces a simple AMD module call "expect" to help us write more
idiomatic unit tests. The API for "expect" is based on the popular Jasmine unit
testing framework for node.js. I investigated just importing one of Node's many
unit testing frameworks, but they all try to do too much (e.g., drive the
entire test harness via Node's file system interface).

The "expect" modules doesn't try to drive the testing process. We just let
gtest handle that. Instead, "expect" just provides a simple language in which
to write test assertions. We'll likely evolve our testing strategy over time,
but hopefully this CL is an improvement over the primitive gtest bindings.

R=jochen@chromium.org
TBR=joth@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/85223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237145 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237145