I used to have the following in my .gclient file:

$ cat .gclient
solutions = [
  {
    "name"        : "src",
    "url"         : "https://chromium.googlesource.com/chromium/src.git",
    "deps_file"   : "DEPS",
    "managed"     : False,
    "custom_deps" : {},
    "safesync_url": "",
  },
]
target_os = ["ios", "mac"]

This allowed me to use the same tree to build for iOS and macOS. However, with hermetic build, I now get failures when trying to generate for mac:

$ gn gen out/tools --args='target_os="mac"'
ERROR at //build/config/mac/mac_sdk.gni:61:5: Script returned non-zero exit code.
    exec_script("//build/mac/find_sdk.py", find_sdk_args, "list lines")
    ^----------
Current dir: /Users/sdefresne/Developer/chromium/src/out/tools/
Command: python -- /Users/sdefresne/Developer/chromium/src/build/mac/find_sdk.py --print_sdk_path --developer_dir /Users/sdefresne/Developer/chromium/src/build/mac_files/Xcode.app 10.10
Returned 1.
stderr:

Traceback (most recent call last):
  File "/Users/sdefresne/Developer/chromium/src/build/mac/find_sdk.py", line 89, in <module>
    print main()
  File "/Users/sdefresne/Developer/chromium/src/build/mac/find_sdk.py", line 57, in main
    sdks = [re.findall('^MacOSX(10\.\d+)\.sdk$', s) for s in os.listdir(sdk_dir)]
OSError: [Errno 2] No such file or directory: '/Users/sdefresne/Developer/chromium/src/build/mac_files/Xcode.app/Platforms/MacOSX.platform/Developer/SDKs'

See //build/toolchain/mac/BUILD.gn:14:1: whence it was imported.
import("//build/config/mac/mac_sdk.gni")
^--------------------------------------
See //BUILD.gn:74:1: which caused the file to be included.
group("gn_all") {
^----------------

Investigating, it looks like the build/mac_files are not downloaded because the script src/build/mac_toolchain.py looks whether "ios" is in target_os but the src/build/mac/find_sdk.py script is still invoked with --developer_dir path pointing to src/build/mac_files.

use_system_xcode is however initialized by invoking build/mac/should_use_hermetic_xcode.py with target_os as a parameter (hence "mac") in my case.

I think we can solve this by changing hermetic_xcode_path from src/build/mac_files/Xcode.app to src/build/${target_os}_files/Xcode.app and modifying src/build/mac_toolchain.py to download ios/mac hermetic builds to src/build/${os}_files. It can use the same condition to detect whether to omit downloading the files for ios.