Checkout chromium on Windows (currently I got revision f49225f43511), run buildtools/checkdeps/checkdeps.py, notice the following errors even though you've made no changes:

$ buildtools/checkdeps/checkdeps.py
Using base directory: C:\src\chromium\src
Checking: C:\src\chromium\src

ERROR in C:\src\chromium\src\ppapi\generators\test_thunk\simple_thunk.cc
  Illegal include: "ppapi\generators\test_thunk\ppapi\test_thunk\simple.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\afl\src\qemu_mode\patches\afl-qemu-cpu-inl.h
  Illegal include: "third_party\afl\src\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\libovr\src\Include\Extras\OVR_CAPI_Util.h
  Illegal include: "third_party\libovr\src\Include\OVR_CAPI.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\libovr\src\Include\Extras\OVR_Math.h
  Illegal include: "third_party\libovr\src\Include\OVR_CAPI.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\chromium\src\config_win.h
  Illegal include: "third_party\tcmalloc\chromium\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\chromium\src\windows\config.h
  Illegal include: "third_party\tcmalloc\chromium\src\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\gperftools-2.0\chromium\src\config_win.h
  Illegal include: "third_party\tcmalloc\gperftools-2.0\chromium\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\gperftools-2.0\chromium\src\windows\config.h
  Illegal include: "third_party\tcmalloc\gperftools-2.0\chromium\src\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\gperftools-2.0\vendor\src\windows\config.h
  Illegal include: "third_party\tcmalloc\gperftools-2.0\vendor\src\config.h"
    Because of no rule applying.

ERROR in C:\src\chromium\src\third_party\tcmalloc\vendor\src\windows\config.h
  Illegal include: "third_party\tcmalloc\vendor\src\config.h"
    Because of no rule applying.

FAILED


I think there should be no errors on a fresh checkout on Windows (I don't have access to Linux/Mac to check whether there are errors on those systems).

This may be due to https://bugs.chromium.org/p/chromium/issues/detail?id=129237 (if checkdeps errors are not counted as errors on Windows, then bot may have left such errors creep in in system specific code).