This git repository exists to allow the source files generated during a Chromium build to be indexed by code search (https://cs.chromium.org).
You do not need to clone this repository, and it is not used in any way during a build.
A build slave regularly does a full Linux build of Chromium, and afterwards pushes all the generated files (from the src/out/Debug/gen directory) into this git repository.
Another build slave maintains a git repository containing gitlinks (git submodules) representing the dependencies specified in Chromium's DEPS file. That script also adds a gitlink named “out” pointing to this repository.
The code search crawler indexes that gitlink repository following all the gitlinks inside, which eventually leads it here and allows it to index the generated code.