Starting with https://build.chromium.org/p/client.webrtc/builders/iOS%20API%20Framework%20Builder/builds/6035 we're seeing errors like this:

FAILED: obj/third_party/boringssl/boringssl/bn_asn1.o 
/b/c/goma_client/gomacc clang -MMD -MF obj/third_party/boringssl/boringssl/bn_asn1.o.d -DV8_DEPRECATION_WARNINGS -DNO_TCMALLOC -DDISABLE_NACL -DCHROMIUM_BUILD -DFIELDTRIAL_TESTING_ENABLED -DCR_CLANG_REVISION=\"310694-2\" -DCR_XCODE_VERSION=0900 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DNS_BLOCK_ASSERTIONS=1 -DBORINGSSL_ALLOW_CXX_RUNTIME -DBORINGSSL_IMPLEMENTATION -DBORINGSSL_NO_STATIC_INITIALIZER -DOPENSSL_SMALL -D_XOPEN_SOURCE=700 -I../.. -Igen -I../../third_party/boringssl/src/include -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -fcolor-diagnostics -arch arm64 -Oz -fno-omit-frame-pointer -gdwarf-2 -isysroot /Applications/Xcode9.0-Beta4.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.0.sdk -stdlib=libc++ -miphoneos-version-min=8.0 -fembed-bitcode -fvisibility=hidden -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Werror -Wall -Wno-unused-variable -Wundeclared-selector -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-covered-switch-default -Wno-unneeded-internal-declaration -Wno-inconsistent-missing-override -Wno-undefined-var-template -Wno-address-of-packed-member -std=c11 -std=c99 -std=c99 -c ../../third_party/boringssl/src/crypto/bn_extra/bn_asn1.c -o obj/third_party/boringssl/boringssl/bn_asn1.o
GOMA:clang compiling ../../third_party/boringssl/src/crypto/bn_extra/bn_asn1.c:*ERROR*: compiler_proxy:1764ms: No matching compiler(clang 4.2.1[Apple LLVM version 9.0.0 (clang-900.0.31)] (51ac8f6fdbe3e5f74576323467f689a99044e38bb95b7321726544264749ff3f)) found in server
GOMA:clang compiling ../../third_party/boringssl/src/crypto/bn_extra/bn_asn1.c:*ERROR*: compiler_proxy:1765ms: /usr/bin/clang is disabled.


It's likely caused by bug 763350 that upgraded these bots to Xcode 9.0 Beta4.
I'm going to try to submit https://codereview.webrtc.org/2995033002/ to start using the hermetic toolchain instead, which might solve this.