Chromium Build System Migrating to Siso

The Chrome Build Infra Team announces that Chromium's build system is switching from Ninja to Siso, a drop-in replacement for Ninja that natively supports remote execution. External developers simply need to continue using autoninja; it will automatically use Siso after running `gn clean` next time. If issues arise, revert to Ninja by setting `use_siso=false` in your `args.gn`. Ninja support ends in late September, along with the removal of Reclient.