I tried to think of a less-unsatisfying answer, but it was just prior knowledge. All the major browsers use Webkit or a fork of WebKit, and WebKit had to be the thing that lays out the DOM since it’s not chrome-specific. (If you were looking for something specific to chrome, it’d probably be a lot harder to find. Or at least for me to find.)

The other comments are good too: try searching the metadata for the code, like git logs, pull requests, etc.

I suppose a new age solution might also be to type “// The DOM layout algorithm:” into Copilot and pray.