Chrome Cloud Messaging

Chrome Cloud Messaging

Implements a basic handler to help register/unregister extensions with Tango, as
well as a basic API test to see if event dispatching works as expected.

This depends on http://codereview.chromium.org/10805002/ to land. Right now, we
just don't register anything.

BUG=135232
TEST=an extension with the <insert permission name here> should receive
notifications associated with it.

This change is probably going to get broken up into at least two smaller changes:
- One containing just the InvalidationHandler changes.
- One containing the plumbing for events so we can have a basic API test.