It looks like when doing fat builds, some deps propagate across toolchain when using ios_framework_bundle target $foo+bundle as can be observed with the following command:

$ ninja -C out/Debug-iphonesimulator -v \
  ios_clang_x86/obj/components/cronet/ios/test/x86/cronet_test
[1/1] TOOL_VERSION=1508804102 ../../build/toolchain/mac/linker_driver.py ../../third_party/llvm-build/Release+Asserts/bin/clang++  -Xlinker -rpath -Xlinker @executable_path/Frameworks -Xlinker -objc_abi_version -Xlinker 2 -Xlinker -sectcreate -Xlinker __TEXT -Xlinker __entitlements -Xlinker obj/components/cronet/ios/test/cronet_test.xcent -arch i386 -Werror -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk -stdlib=libc++ -mios-simulator-version-min=9.0 -Wl,-ObjC -F gen/components/cronet/ios -Lgen/components/cronet/ios -o "ios_clang_x86/obj/components/cronet/ios/test/x86/cronet_test" -Wl,-filelist,"ios_clang_x86/obj/components/cronet/ios/test/x86/cronet_test.rsp"  -framework UIKit -framework CoreFoundation -framework CoreGraphics -framework CoreText -framework Foundation -framework Cronet -framework CFNetwork -framework MobileCoreServices -framework Security -framework SystemConfiguration -lresolv
ld: warning: ignoring file obj/components/grpc_support/grpc_support/bidirectional_stream_c.o, file was built for x86_64 which is not the architecture being linked (i386): obj/components/grpc_support/grpc_support/bidirectional_stream_c.o
ld: warning: ignoring file obj/components/grpc_support/grpc_support/bidirectional_stream.o, file was built for x86_64 which is not the architecture being linked (i386): obj/components/grpc_support/grpc_support/bidirectional_stream.o
ld: warning: ignoring file obj/net/constants/trace_constants.o, file was built for x86_64 which is not the architecture being linked (i386): obj/net/constants/trace_constants.o
ld: warning: ignoring file obj/base/libbase_static.a, file was built for archive which is not the architecture being linked (i386): obj/base/libbase_static.a
ld: warning: ignoring file obj/base/third_party/dynamic_annotations/libdynamic_annotations.a, file was built for archive which is not the architecture being linked (i386): obj/base/third_party/dynamic_annotations/libdynamic_annotations.a
ld: warning: ignoring file obj/third_party/modp_b64/libmodp_b64.a, file was built for archive which is not the architecture being linked (i386): obj/third_party/modp_b64/libmodp_b64.a
ld: warning: ignoring file obj/base/third_party/libevent/libevent.a, file was built for archive which is not the architecture being linked (i386): obj/base/third_party/libevent/libevent.a

If I remove the dependency on //components/cronet/ios:cronet_framework+link from //components/cronet/ios/test:cronet_test, then the error vanish (the build fail due to missing symbols, but this is expected). Looking at the .rsp file, I can see the following:

$ cat out/Debug-iphonesimulator/ios_clang_x86/obj/components/cronet/ios/test/x86/cronet_test.rsp
...
ios_clang_x86/obj/net/simple_quic_tools/quic_time_wait_list_manager.o
ios_clang_x86/obj/net/simple_quic_tools/stateless_rejector.o
ios_clang_x86/obj/net/simple_quic_tools/synchronous_host_resolver.o
obj/components/grpc_support/grpc_support/bidirectional_stream.o
obj/components/grpc_support/grpc_support/bidirectional_stream_c.o
obj/net/constants/trace_constants.o
...

So we try to link an binary for a secondary toolchain, but the dependency from primary toolchain leak out. This is probably due to the fact that the +link target do depends on both the primary and secondary target due to the "lipo" target. I'm not really sure how to prevent as we want the deps to work, but we'd like to avoid the libs to propagate.

Since it is a warning, this is not blocking, so leaving a Pri-3.