I discovered today that if your target has a dependency on //base/test:test_support, then the flag -Wno-unused-function is added to the command-line and thus disabling errors about unused functions (at least on iOS).

Some investigation showed that this is caused by the following:

$ gn desc out/default //base/test:test_support configs
//build/config:feature_flags
//build/config/compiler:afdo
//build/config/compiler:compiler
//build/config/compiler:clang_stackrealign
//build/config/compiler:compiler_arm_fpu
//build/config/compiler:chromium_code
//build/config/compiler:default_include_dirs
//build/config/compiler:default_optimization
//build/config/compiler:default_stack_frames
//build/config/compiler:default_symbols
//build/config/compiler:no_rtti
//build/config/compiler:runtime_library
//build/config/sanitizers:default_sanitizer_flags
//build/config/gcc:no_exceptions
//build/config/gcc:symbol_visibility_hidden
//build/config/clang:find_bad_constructs
//build/config/clang:extra_warnings
//build/config:debug
//build/config:precompiled_headers
//testing/gtest:gtest_config
//third_party/ced:ced_config
//third_party/icu:icu_config
//testing/gmock:gmock_config
//testing/gtest:gtest_direct_config
//third_party/libxml:libxml_config

$ for config in $(gn desc out/default //base/test:test_support configs); do
    if gn desc out/default $config|grep -q Wno-unused-function; then
      echo $config;
    fi;
  done
//third_party/ced:ced_config

$ gn desc out/default //third_party/ced:ced_config
Config: //third_party/ced:ced_config
Toolchain: //build/toolchain/mac:ios_clang_x64

cflags
  -Wno-unused-function

include_dirs
  //third_party/ced/src/

I think this config should be split in two, with the cflags limited to the //third_party/cec and the include_dirs exported as public_configs. This may requires fixing many test targets that currently get this dependency, so created a bug to track this.