What steps will reproduce the problem?
(1) Install MSVC 2017. Currently only available is 15.7.2 AFAIK 
(2) gn gen


What is the expected result?

gn gen succeeds

What happens instead?

c:\Users\kkinnunen\goma\client>set DEPOT_TOOLS_WIN_TOOLCHAIN=0
set DEPOT_TOOLS_WIN_TOOLCHAIN=0

c:\Users\kkinnunen\goma\client>gn gen --args=is_debug=false out/Release
gn gen --args=is_debug=false out/Release
Traceback (most recent call last):
  File "c:/Users/kkinnunen/goma/client/build/toolchain/win/setup_toolchain.py", line 204, in <module>
    main()
  File "c:/Users/kkinnunen/goma/client/build/toolchain/win/setup_toolchain.py", line 172, in main
    env = _LoadToolchainEnv(cpu, win_sdk_path)
  File "c:/Users/kkinnunen/goma/client/build/toolchain/win/setup_toolchain.py", line 133, in _LoadToolchainEnv
    script_path)
Exception: C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\vcvarsall.bat is missing - make sure VC++ tools are installed.
ERROR at //build/toolchain/win/BUILD.gn:351:22: Script returned non-zero exit code.
x64_toolchain_data = exec_script("setup_toolchain.py",
                     ^----------
Current dir: c:/Users/kkinnunen/goma/client/out/Release/
Command: c:/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe -- c:/Users/kkinnunen/goma/client/build/toolchain/win/setup_toolchain.py "C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional" "C:\Program Files (x86)\Windows Kits\10" "C:\Windows\Sysnative;C:\Windows\SysWOW64" x64 /I
Returned 1.
See //BUILD.gn:5:1: which caused the file to be included.
group("root") {
^--------------

In my version, the script is in:
C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Auxiliary\Build

IIRC the current Chromium vs_toolchain.py works with the later versions of VS