Previously win_more_configs (a bot which compiles 8 different configurations) was flaky.
Example failure: https://build.chromium.org/p/tryserver.webrtc/builders/win_more_configs/builds/4301
This happened because ninja's caching is buggy on Windows, and all those different configurations were built in the same directory. 

Solution: Build each config in a different out dir.
https://chromium-review.googlesource.com/c/chromium/tools/build/+/883365
This worked out well in terms of flakiness, but now there is a new problem: it's much slower.
Apparently, rebuilding with a different config is typically faster than rebuilding with modified source code. We definitely don't want to go back to the flaky compilation, but if win_more_configs takes 50 minutes on average when most bots are under 25 minutes, that's a problem.

Other _more_configs bots also became slower, by the way, and they didn't even necessarily have to change to using different out dirs.