Using latest file to determine latest version for lacros runner

The latest/ash-chromium.txt contains the latest version number.

Bug: 1109287
Change-Id: I1797c00ee22720eceb4ea3b43c8d29717b0e6f92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343544
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796567}

build/lacros/test_runner.py

@@ -55,6 +55,9 @@ sys.path.append(os.path.join(_SRC_ROOT, 'third_party', 'depot_tools'))
 # Base GS URL to store prebuilt ash-chrome.
 _GS_URL_BASE = 'gs://ash-chromium-on-linux-prebuilts/x86_64'
 
+# Latest file version.
+_GS_URL_LATEST_FILE = _GS_URL_BASE + '/latest/ash-chromium.txt'
+
 # GS path to the zipped ash-chrome build with any given version.
 _GS_ASH_CHROME_PATH = 'ash-chromium.zip'
 
@@ -198,12 +201,10 @@ def _GetLatestVersionOfAshChrome():
   gsutil = download_from_google_storage.Gsutil(
       download_from_google_storage.GSUTIL_DEFAULT_PATH)
 
-  # gsutil.check_call returns (code, out, err), and each path in the output is
-  # of format: gs://ash-chromium-on-linux-prebuilts/x86_64/{cr-commit-position}
-  _, output, _ = gsutil.check_call('ls', _GS_URL_BASE)
-  latest_gs_path = sorted(output.splitlines())[-1]
-  return latest_gs_path.rstrip('/').split('/')[-1]
-
+  with tempfile.NamedTemporaryFile() as tmp:
+    gsutil.check_call('cp', _GS_URL_LATEST_FILE, tmp.name)
+    with open(tmp.name, 'r') as f:
+      return f.read().strip()
 
 def _RunTestWithAshChrome(args, forward_args):
   """Runs tests with ash-chrome.