UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.20515 Safari/537.36 Sparrow/0.1

Steps to reproduce the problem:
1. Followed instructions at https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md
2. Ran command 'gn gen out/Default'
3. Saw the error

What is the expected behavior?
The command should succeed.

What went wrong?
The gn gen command failed with the following output:
E:\Work\workspace0\src>gn gen out\Release
Traceback (most recent call last):
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 476, in <module>
    sys.exit(main())
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 472, in main
    return commands[sys.argv[1]](*sys.argv[2:])
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 295, in CopyDlls
    _CopyRuntime(target_dir, runtime_dir, target_cpu, debug=True)
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 275, in _CopyRuntime

    suffix)
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 211, in _CopyUCRTRun
time
    os.path.join(source_dir, 'ucrtbase' + suffix))
  File "E:/Work/workspace0/src/build/vs_toolchain.py", line 179, in _CopyRuntime
Impl
    shutil.copy2(source, target)
  File "C:\Users\jtsillas\Git\depot_tools\win_tools-2_7_6_bin\python\bin\lib\shu
til.py", line 130, in copy2
    copyfile(src, dst)
  File "C:\Users\jtsillas\Git\depot_tools\win_tools-2_7_6_bin\python\bin\lib\shu
til.py", line 82, in copyfile
    with open(src, 'rb') as fsrc:
IOError: [Errno 2] No such file or directory: 'C:\\WINDOWS/SysWOW64\\ucrtbased.d
ll'
ERROR at //build/toolchain/win/BUILD.gn:43:3: Script returned non-zero exit code
.
  exec_script("../../vs_toolchain.py",
  ^----------
Current dir: E:/Work/workspace0/src/out/Release/
Command: C:/Users/jtsillas/Git/depot_tools/win_tools-2_7_6_bin/python/bin/python
.exe E:/Work/workspace0/src/build/vs_toolchain.py copy_dlls E:/Work/workspace0/s
rc/out/Release Debug x86
Returned 1 and printed out:

Copying C:\WINDOWS/SysWOW64\ucrtbased.dll to E:/Work/workspace0/src/out/Release\
ucrtbased.dll...

See //BUILD.gn:61:1: which caused the file to be included.
group("gn_all") {
^----------------
Traceback (most recent call last):
  File "E:/Work/workspace0/src/build/toolchain/win/setup_toolchain.py", line 292
, in <module>
    main()
  File "E:/Work/workspace0/src/build/toolchain/win/setup_toolchain.py", line 284
, in main
    assert vc_lib_um_path
AssertionError

Did this work before? Yes 68

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