Unit test utils for Encoder and Decoder.

The tests will cover the state and messages sent out of the Encoder and
Decoder are in correct order.

It also make sure Decoded output matches input to the Encoder.

Review URL: http://codereview.chromium.org/3011010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@53566 0039d316-1c4b-4281-b951-d872f2087c98