I added an FALLTHROUGH macro to base/compiler_specific. The trybots came back red because apparently some files include webrtc's version and base's, due to webrtc_overrides:

https://ci.chromium.org/buildbot/tryserver.chromium.linux/cast_shell_linux/528094

FAILED: obj/third_party/webrtc/p2p/rtc_p2p/basicpacketsocketfactory.o 
/b/c/goma_client/gomacc ../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/third_party/webrtc/p2p/rtc_p2p/basicpacketsocketfactory.o.d -DV8_DEPRECATION_WARNINGS -DDCHECK_ALWAYS_ON=1 -DUSE_AURA=1 -DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DCHROMIUM_BUILD -DFIELDTRIAL_TESTING_ENABLED -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -DCR_CLANG_REVISION=\"321529-2\" -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DWEBRTC_ENABLE_PROTOBUF=1 -DWEBRTC_RESTRICT_LOGGING -DHAVE_SCTP -DENABLE_EXTERNAL_AUTH -DHAVE_WEBRTC_VIDEO -DHAVE_WEBRTC_VOICE -DLOGGING_INSIDE_WEBRTC -DUSE_WEBRTC_DEV_BRANCH -DWEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS=0 -DFEATURE_ENABLE_VOICEMAIL -DGTEST_RELATIVE_PATH -DWEBRTC_CHROMIUM_BUILD -DWEBRTC_POSIX -DWEBRTC_LINUX -DFEATURE_ENABLE_VOICEMAIL -DNO_MAIN_THREAD_WRAPPING -I../.. -Igen -I../../third_party/webrtc_overrides -I../../third_party/webrtc -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -funwind-tables -fPIC -pipe -B../../third_party/binutils/Linux_x64/Release/bin -pthread -fcolor-diagnostics -Xclang -mllvm -Xclang -instcombine-lower-dbg-declare=0 -fdebug-prefix-map=/b/c/b/cast_shell_linux/src/out/Release=. -no-canonical-prefixes -m64 -march=x86-64 -Wall -Werror -Wextra -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-nonportable-include-path -Wno-address-of-packed-member -Wno-unused-lambda-capture -Wno-user-defined-warnings -Wno-enum-compare-switch -Wno-tautological-unsigned-zero-compare -Wno-null-pointer-arithmetic -Wno-tautological-constant-compare -Wtautological-constant-out-of-range-compare -O2 -fno-ident -fdata-sections -ffunction-sections -fno-omit-frame-pointer -g1 -fvisibility=hidden -Xclang -load -Xclang ../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so -Xclang -add-plugin -Xclang find-bad-constructs -Xclang -plugin-arg-find-bad-constructs -Xclang check-ipc -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -std=gnu++14 -fno-exceptions -fno-rtti -nostdinc++ -isystem../../buildtools/third_party/libc++/trunk/include -isystem../../buildtools/third_party/libc++abi/trunk/include --sysroot=../../build/linux/debian_stretch_amd64-sysroot -fvisibility-inlines-hidden -c ../../third_party/webrtc/p2p/base/basicpacketsocketfactory.cc -o obj/third_party/webrtc/p2p/rtc_p2p/basicpacketsocketfactory.o
In file included from ../../third_party/webrtc/p2p/base/basicpacketsocketfactory.cc:21:
In file included from ../../third_party/webrtc/rtc_base/nethelpers.h:24:
In file included from ../../third_party/webrtc/rtc_base/signalthread.h:18:
In file included from ../../third_party/webrtc/rtc_base/nullsocketserver.h:14:
In file included from ../../third_party/webrtc_overrides/rtc_base/event.h:9:
In file included from ../../base/synchronization/waitable_event.h:30:
In file included from ../../base/memory/ref_counted.h:14:
../../base/compiler_specific.h:226:9: error: 'FALLTHROUGH' macro redefined [-Werror,-Wmacro-redefined]
#define FALLTHROUGH [[clang::fallthrough]]
        ^
../../third_party/webrtc/typedefs.h:98:9: note: previous definition is here
#define FALLTHROUGH() [[clang::fallthrough]]
        ^


In this concrete case it's not the end of the world (I can just add an #if !defined(FALLTHROUGH)) but abstractly this is pretty big problem and kind of violates the "things not in src.git shouldn't depend on base" maxim (since webrtc now indirectly, via webrtc_overrides, does depend on base.)

Maybe it's possible to implement webrtc_overrides so that no _header_ file includes stuff from base?