Switch to the Chrome Stable channel (i.e., Chrome 27)

Up until now we've always been building `trunk` revisions of Chromium. This is kinda cool because it lets us use the latest web features right away, but kinda uncool because it exposes us to a bunch of bugs that get found and fixed when Chrome stabilizes before a release. Plus, I haven't been updating the underlying version of Chromium often enough to really see much benefit from building bleeding-edge revisions.

So now we build the latest Stable channel release of Chrome (i.e., Chrome 27). I think it might be useful to have a branch that still builds bleeding-edge revisions (or maybe Beta/Dev/Canary releases?) so that we can adapt to changes to the Chromium build process/API incrementally, but the stability of the Stable channel is desirable for most uses.