From https://build.chromium.org/p/chromium.fyi/builders/ClangToTWin%28dbg%29/builds/7561/steps/compile/logs/stdio:


../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes /FC @obj/third_party/webrtc/base/rtc_base/thread.obj.rsp /c ../../third_party/webrtc/base/thread.cc /Foobj/third_party/webrtc/base/rtc_base/thread.obj /Fd"obj/third_party/webrtc/base/rtc_base_cc.pdb"
C:\b\c\b\ClangToTWin_dbg_\src\third_party\webrtc\base\thread.cc(231,35):  error: cast between incompatible calling conventions 'cdecl' and 'stdcall'; calls through this pointer may abort at runtime [-Werror,-Wcast-calling-convention]
  thread_ = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)PreRun, init, 0,
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\b\c\b\ClangToTWin_dbg_\src\third_party\webrtc\base\thread.h(241,16):  note: consider defining 'PreRun' with the 'stdcall' calling convention
  static void *PreRun(void *pv);
               ^
               STDAPICALLTYPE 




I don't see this on the pinned bots, so might be due to a Clang change.