There is a build error on win with following gn args:
target_cpu = "x86"
enable_nacl = false
is_component_build = true
is_debug = true
use_lld = false
win_console_app = true
use_goma = true

Here is output:
$ autoninja services/media_session/public/mojom/
ninja.exe services/media_session/public/mojom/ -j 1120 -l 56
[1/1] Regenerating ninja files
[2/2] LINK(DLL) media_session_public_mojom.dll media_ses..._public_mojom.dll.lib media_session_public_mojom.dll.pdb
FAILED: media_session_public_mojom.dll media_session_public_mojom.dll.lib media_session_public_mojom.dll.pdb
C:/src/build/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe ../../build/toolchain/win/tool_wrapper.py link-wrapper environment.x86 False link.exe /nologo /IMPLIB:./media_session_public_mojom.dll.lib /DLL /OUT:./media_session_public_mojom.dll /PDB:./media_session_public_mojom.dll.pdb @./media_session_public_mojom.dll.rsp
LINK : fatal error LNK1104: cannot open file 'media_session_public_mojom_shared.dll.lib'
ninja: build stopped: subcommand failed.
exit status 1

audio_focus.mojom:
module media_session.mojom;
enum AudioFocusType {
  kGain,
  kGainTransientMayDuck,
};

It has only enum and it should not create dll or so.

This is really similar to https://bugs.chromium.org/p/chromium/issues/detail?id=848134
but BUILD.gn uses mojom_component macro and I'm not sure how might we fix this.