3. Firefox fades into obscurity/irrelevance, Chromium becomes the platform of the web, people realize this and forks abound, and eventually someone makes it into an ECMA standard and Google de facto loses control of it.
Oh also, another one:
4. The Linux Foundation or some other entity steps in and just builds a new GPL'd browser, which can mimic the success story of the Linux kernel (where the copyleft license will ensure that the vast number of open-source hackers prefer to contribute to it over something like the BSD-licensed Chromium).