For https://chromium-review.googlesource.com/c/558251/ I triggered some perf trybots, and some of them fail to build with the following patch error:

error: Your local changes to the following files would be overwritten by checkout:
	third_party/WebKit/Source/core/testing/data/mhtml/simple_test.mht
Please commit your changes or stash them before you switch branches.
Aborting

See for example:
https://build.chromium.org/p/tryserver.chromium.perf/builders/win_x64_perf_bisect/builds/1863

I don't see how my CL could be causing that.