Cross-compiling Chrome/win

It's possible to build most 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:

goma. Sorry. (internal bug) You can use the jumbo build for faster build times.
mini_installer (bug)
linking on Mac (bug), should change soon

All other targets build fine (including chrome, browser_tests, ...).

Uses of .asm files have been stubbed out. As a result, some of Skia's software rendering paths are not present in cross builds, Crashpad cannot report crashes, and NaCl defaults to disabled and cannot be enabled in cross builds (.asm bug).

.gclient setup

Tell gclient that you need Windows build dependencies by adding 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.) e.g.
```
solutions = [
  {
    ...
  }
]
target_os = ['android', 'win']
```
gclient sync, follow instructions on screen.

If you're at Google, this will automatically download the Windows SDK for you. If this fails with an error: Please follow the instructions at https://www.chromium.org/developers/how-tos/build-instructions-windows then you may need to re-authenticate via:

cd path/to/chrome/src
# Follow instructions, enter 0 as project id.
download_from_google_storage --config

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.

GN setup

Add target_os = "win" to your args.gn. Then just build, e.g.

ninja -C out/gnwin base_unittests.exe

Running tests on swarming

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>
    [ -- <flag to target process, for example --gtest_filter>... ]

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).