https://build.chromium.org/p/chromium.fyi/builders/CrWinClangGoma/builds/22984/steps/compile/logs/stdio

FAILED: obj/third_party/WebKit/Source/core/core_generated/ScriptWrappableVisitor.obj 
E:\b\c\goma_client/gomacc.exe ../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes /FC @obj/third_party/WebKit/Source/core/core_generated/ScriptWrappableVisitor.obj.rsp /c ../../third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp /Foobj/third_party/WebKit/Source/core/core_generated/ScriptWrappableVisitor.obj /Fd"obj/third_party/WebKit/Source/core/core_generated_cc.pdb"
In file included from ../../third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp:10:
In file included from ../../third_party/WebKit/Source\bindings/core/v8/ScriptWrappableVisitorVerifier.h:11:
In file included from ../../third_party/WebKit/Source\core/dom/ElementRareData.h:26:
In file included from ../../third_party/WebKit/Source\core/animation/ElementAnimations.h:36:
In file included from ../../third_party/WebKit/Source\core/animation/css/CSSAnimations.h:40:
In file included from ../../third_party/WebKit/Source\core/css/resolver/StyleResolverState.h:37:
In file included from ../../third_party/WebKit/Source\core/style/CachedUAStyle.h:26:
In file included from ../../third_party/WebKit/Source\core/style/ComputedStyle.h:52:
In file included from ../../third_party/WebKit/Source\core/style/StyleRareNonInheritedData.h:30:
In file included from ../../third_party/WebKit/Source\core/style/ClipPathOperation.h:33:
E:\b\c\b\CrWinClangGoma\src\third_party\WebKit\Source\core\style\BasicShapes.h(222,1):  error: implicit conversion from 'int' to 'unsigned char' changes value from 256 to 0 [-Werror,-Wconstant-conversion]
DEFINE_BASICSHAPE_TYPE_CASTS(BasicShapeEllipse);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
E:\b\c\b\CrWinClangGoma\src\third_party\WebKit\Source\core\style\BasicShapes.h(77,3):  note: expanded from macro 'DEFINE_BASICSHAPE_TYPE_CASTS'
  DEFINE_TYPE_CASTS(thisType, BasicShape, value,                 \
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
E:\b\c\b\CrWinClangGoma\src\third_party\WebKit\Source\wtf\Assertions.h(244,5):  note: expanded from macro 'DEFINE_TYPE_CASTS'
    CHECK(!argument || (pointerPredicate));                                   \
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
E:\b\c\b\CrWinClangGoma\src\base\logging.h(540,28):  note: expanded from macro 'CHECK'
  UNLIKELY(!(condition)) ? IMMEDIATE_CRASH() : EAT_STREAM_PARAMETERS
                           ^~~~~~~~~~~~~~~~~
E:\b\c\b\CrWinClangGoma\src\base\logging.h(515,59):  note: expanded from macro 'IMMEDIATE_CRASH'
   (void)(*reinterpret_cast<volatile unsigned char*>(0) = __COUNTER__))
                                                        ~ ^~~~~~~~~~~
<scratch space>(160,1):  note: expanded from here
256
^~~

seems https://chromium.googlesource.com/chromium/src/+/92bbdc39154ce7cbbc939d9a4c5ee371691ab811 is culprit.