Use your favorite GYP_DEFINES but include target_arch=ia32:

$ GYP_DEFINES="disable_nacl=1 fastbuild=1 target_arch=ia32 component=shared_library use_goma=1" build/gyp_chromium
Updating projects from gyp files...


Then build any target that invokes protoc, e.g.

$ ninja -C out/Release/ -j 1000 ../../third_party/cros_system_api/dbus/mtp_file_entry.proto^
ninja: Entering directory `out/Release/'
[1/1] RULE Generating C++ and Python code from ../../third_party/cros_system_api/dbus/mtp_file_entry.proto
FAILED: cd ../../device/media_transfer_protocol; python ../../tools/protoc_wrapper/protoc_wrapper.py --include "" --protobuf "../../out/Release/gen/protoc_out/device/media_transfer_protocol/mtp_file_entry.pb.h" --proto-in-dir ../../third_party/cros_system_api/dbus --proto-in-file "mtp_file_entry.proto" "--use-system-protobuf=0" -- ../../out/Release/protoc --cpp_out ../../out/Release/gen/protoc_out/device/media_transfer_protocol --python_out ../../out/Release/pyproto/device/media_transfer_protocol
terminate called after throwing an instance of 'std::length_error'
  what():  vector::_M_range_insert
ninja: build stopped: subcommand failed.