It's possible to build parts of the codebase on a Linux (and soon, Mac) host while targeting Windows. This document describes how to set that up, and current restrictions.
What does not work:
This disqualifies most interesting targets for now, but a few smaller ones (base_unittests, ...) do work. Over time, more things should work.
target_os = ['win'] to the end of your .gclient. (If you already have a target_os line in there, just add 'win' to the list.)gclient sync, follow instructions on screen.If you‘re at Google, this will automatically download the Windows SDK for you. If you are not at Google, you’ll have to figure out how to get the SDK, and you'll need to put a JSON file describing the SDK layout in a certain location.
Add target_os = "win" to your args.gn. Then just build, e.g.
ninja -C out/gnwin base_unittests.exe
You can run the Windows binaries you built on swarming, like so:
tools/mb/mb.py isolate //out/gnwin base_unittests
tools/swarming_client/isolate.py archive \
-I https://isolateserver.appspot.com \
-i out/gnwin/base_unittests.isolate \
-s out/gnwin/base_unittests.isolated
tools/swarming_client/swarming.py trigger \
-S https://chromium-swarm.appspot.com \
-I https://isolateserver.appspot.com \
-d os Windows -d pool Chrome -s <hash printed by previous command>
Most tests that build should pass. However, the cross build uses the lld linker, and a couple of tests fail when using lld. You can look at https://build.chromium.org/p/chromium.clang/builders/CrWinClangLLD%20tester to get an idea of which tests fail with lld.
TODO(thakis): It'd be nice if there was a script for doing this. Maybe make tools/fuchsa/run-swarmed.py work for win cross builds too, or create run_base_unittests script targets during the build (like Android).