I recently tried a Chromium build @ p491708 and got the following error message:

ninja: Entering directory `out/Default'
[1/1] Regenerating ninja files
FAILED: build.ninja 
/Users/rdsmith/Sandboxen/chrome/src/buildtools/mac/gn --root=/Users/rdsmith/Sandboxen/chrome/src -q gen .
ERROR at //build/config/mac/mac_sdk.gni:55:5: Script returned non-zero exit code.
    exec_script("//build/mac/find_sdk.py", find_sdk_args, "list lines")
    ^----------
Current dir: /Users/rdsmith/Sandboxen/chrome/src/out/Default/
Command: python -- /Users/rdsmith/Sandboxen/chrome/src/build/mac/find_sdk.py --print_sdk_path --developer_dir /Users/rdsmith/Sandboxen/chrome/src/build/mac_files/Xcode.app 10.12
Returned 1.
stderr:

Traceback (most recent call last):
  File "/Users/rdsmith/Sandboxen/chrome/src/build/mac/find_sdk.py", line 89, in <module>
    print main()
  File "/Users/rdsmith/Sandboxen/chrome/src/build/mac/find_sdk.py", line 62, in main
    raise Exception('No %s+ SDK found' % min_sdk_version)
Exception: No 10.12+ SDK found

See //build/config/sysroot.gni:74:3: whence it was imported.
  import("//build/config/mac/mac_sdk.gni")
  ^--------------------------------------
See //base/BUILD.gn:28:1: whence it was imported.
import("//build/config/sysroot.gni")
^----------------------------------
See //BUILD.gn:72:5: which caused the file to be included.
    "//base:base_unittests",
    ^----------------------
ninja: error: rebuilding 'build.ninja': subcommand failed

The find_sdk.py script was given an argument of /Users/rdsmith/Sandboxen/chrome/src/build/mac_files/Xcode.app 10.12, which makes me think that the files needed should have been automatically downloaded by a gclient sync, but they weren't.  Either the script is giving a confusing and incomplete error message, or there's some problem with gclient sync that is not downloading the correct libraries.