Repro steps for me is to work a while, then git pull --rebase and gclient sync about a week later.

[2068/22923] ACTION //remoting/webapp:webapp(//build/toolchain/win:win_clang_x86)
FAILED: remoting-webapp.v2.zip
C:/src/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe ../../remoting/webapp/build-webapp.py Dev 64.0.3256.0 remoting/remoting-webapp.v2 remoting-webapp.v2.zip ../../remoting/webapp/crd/manifest.json.jinja2 --files_listfile gen/remoting/webapp/webapp_files.txt --locales_listfile gen/remoting/webapp/webapp_locales.txt --use_gcd 0
Traceback (most recent call last):
  File "../../remoting/webapp/build-webapp.py", line 380, in <module>
    sys.exit(main())
  File "../../remoting/webapp/build-webapp.py", line 376, in main
    return buildWebApp(**vars(args))
  File "../../remoting/webapp/build-webapp.py", line 233, in buildWebApp
    replaceString(destination, 'DIRECTORY_API_BASE_URL', directoryApiBaseUrl)
  File "../../remoting/webapp/build-webapp.py", line 63, in replaceString
    "'" + placeholder + "'", "'" + value + "'")
  File "../../remoting/webapp/build-webapp.py", line 41, in findAndReplace
    os.rename(filepath, oldFilepath)
WindowsError: [Error 183] Cannot create a file when that file already exists
[2227/22923] CXX obj/components/policy/chrome_settings_proto_generated_compile_proto/chrome_settings.pb.obj

os.rename python documentation says: On Windows, if dst already exists, OSError will be raised even if it is a file; there may be no way to implement an atomic rename when dst names an existing file.

.. so filepath probably has to be removed if the file already exists, which is what I end up doing to mitigate this build error.