REPRO STEPS:

1. Get Windows 10 machine
2. Get or find Chromium sources
3. Start with no out/rel directory (or delete it)
4. Setup GN:
      D:\src\chromium\src>type out\rel\args.gn
      is_debug = false
      symbol_level = 0
      is_clang = true
5. Build everything:
      D:\src\chromium\src>ninja -j100 -l36 -d keeprsp -C out/rel

EXPECTED: Everything builds fine.

ACTUAL BEHAVIOR: Build fails with:

FAILED: obj/content/app/content_main_runner_browser/content_main_runner.obj
../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes /FC @obj/content/app/content_main_runner_browser/content_main_runner.obj.rsp /c ../../content/app/content_main_runner.cc /Foobj/content/app/content_main_runner_browser/content_main_runner.obj /Fd"obj/content/app/content_main_runner_browser_cc.pdb"
In file included from ../../content/app/content_main_runner.cc:49:
In file included from ../..\content/browser/gpu/gpu_process_host.h:32:
D:\src\chromium\src\out\rel\gen\services\ui\gpu\interfaces\gpu_host.mojom.h(36,10):  fatal error: 'services/ui/gpu/interfaces/context_lost_reason.mojom.h' file not found
#include "services/ui/gpu/interfaces/context_lost_reason.mojom.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

FWIW, I can manually request building of the missing file (or "files" actually) and then successfully build the object file that failed above.  This seems to indicate a missing build dependencies between the object file and the missing file.  Commands to build the missing file and to try rebuilding the object file:

D:\src\chromium\src>ninja -j100 -l36 -d keeprsp -C out/rel gen/services/ui/gpu/interfaces/context_lost_reason.mojom.h
ninja: Entering directory `out/rel'
[1/1] ACTION //services/ui/gpu/interfaces:interfaces__generator(//build/toolchain/win:clang_x64)

D:\src\chromium\src>ninja -j100 -l36 -d keeprsp -C out/rel gen/services/ui/gpu/interfaces/gpu_main.mojom.h
ninja: Entering directory `out/rel'
[1/1] ACTION //services/ui/gpu/interfaces:interfaces__generator(//build/toolchain/win:clang_x64)

D:\src\chromium\src>ninja -j100 -l36 -d keeprsp -C out/rel gen/services/ui/gpu/interfaces/gpu_service.mojom.h
ninja: Entering directory `out/rel'
[1/1] ACTION //services/ui/gpu/interfaces:interfaces__generator(//build/toolchain/win:clang_x64)

D:\src\chromium\src>ninja -j100 -l36 -d keeprsp -C out/rel obj/content/app/content_main_runner_browser/content_main_runner.obj
ninja: Entering directory `out/rel'
[1/1] CXX obj/content/app/content_main_runner_browser/content_main_runner.obj