The PFQ builders amd64-generic-chromium-pfq, arm-generic-chromium-pfq, and daisy-chromium-pfq have begun failing in TestSimpleChromeWorkflow (the other PFQ builders don't get to that step due to issue 769369).

It seems the chrome checkout can't be found. Sample failure: https://uberchromegw.corp.google.com/i/chromeos/builders/amd64-generic-chromium-pfq/builds/10665

Unlike good builds, in these failures the args.gn file does not exist where the builder expects it:

```
23:36:32: WARNING: No args.gn file: /b/c/cbuild/repository/.cache/distfiles/target-master/chrome-src/src/out_amd64-generic/Release/args.gn
```

Then it runs `gn gen`, which fails with:

```
gn.py: Could not find checkout in any parent of the current path.
This must be run inside a checkout.
23:36:32: ERROR: cros chrome-sdk failed before completing.
```

Stack trace:

```
Traceback (most recent call last):
  File "/b/c/cbuild/repository/chromite/bin/cros", line 168, in <module>
    DoMain()
  File "/b/c/cbuild/repository/chromite/bin/cros", line 164, in DoMain
    commandline.ScriptWrapperMain(FindTarget)
  File "/b/c/cbuild/repository/chromite/lib/commandline.py", line 910, in ScriptWrapperMain
    ret = target(argv[1:])
  File "/b/c/cbuild/repository/chromite/scripts/cros.py", line 65, in main
    code = _RunSubCommand(subcommand)
  File "/b/c/cbuild/repository/chromite/scripts/cros.py", line 50, in _RunSubCommand
    return subcommand.Run()
  File "/b/c/cbuild/repository/chromite/cli/cros/cros_chrome_sdk.py", line 1052, in Run
    goma_dir=goma_dir, goma_port=goma_port)
  File "/b/c/cbuild/repository/chromite/cli/cros/cros_chrome_sdk.py", line 874, in _SetupEnvironment
    self._UpdateGnArgsIfStale(gn_args, gn_args_file_path, env['SDK_BOARD'])
  File "/b/c/cbuild/repository/chromite/cli/cros/cros_chrome_sdk.py", line 652, in _UpdateGnArgsIfStale
    capture_output=False)
  File "/b/c/cbuild/repository/chromite/lib/cros_build_lib.py", line 653, in RunCommand
    raise RunCommandError(msg, cmd_result)
chromite.lib.cros_build_lib.RunCommandError: return code: 1; command: gn gen out_amd64-generic/Release '--args=use_ozone = true
...
```