Chromium being open-source is a red herring here. The web is not an application, it is a protocol between clients and servers. Google has immense influence on both sides of that protocol, and regularly attempts to use that influence in an anticompetitive way. Chromium being open source just means you can fork the client, but that gives you essentially no influence over the web as a protocol; if Google wants to take the web in a direction that your fork doesn't agree with (e.g. adding new APIs that you don't want to support or getting rid of APIs that you do), then you're out of luck regardless of your ability to compile the code.

What are the two sides of the protocol?