To be fair, Google Chrome is completely open source too, expect for the integrated PDF Reader and Flash. You can compile Chromium from the source and add these two manually, if you need them.
It's a bit of both with a healthy sprinkling of Nokia, Samsung, and quite a few others. In reality, WebKit has been driving the standards process for quite some time now (especially considering its unrivaled dominance on mobile).
It's Google that decides what features and protocols are implemented into Chrome. Since they control a large share of the market, it basically gives them life or death power on new features of web languages.
