This is a mix of commit fd011815c455976b15e31966f826628b4f9f61d4
("don't keep use_reclient=true with .reproxy_tmp") pulled from upstream
plus a revert of upstream's 49b23faa16ad14e96601aea8772c7279fcbd6b44
("sysroot.gni: Use path_exists() rather than exec_script("dir_exists.py")
in order to work on older gn that lacks path_exists().

--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -61,7 +61,9 @@ if (sysroot == "") {
         _script_arch = "amd64"
       }
       assert(
-          path_exists(sysroot),
+          exec_script("//build/dir_exists.py",
+                      [ rebase_path(sysroot) ],
+                      "string") == "True",
           "Missing sysroot ($sysroot). To fix, run: build/linux/sysroot_scripts/install-sysroot.py --arch=$_script_arch")
     }
   } else if (is_mac) {
--- /dev/null
+++ b/build/dir_exists.py
@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+# Copyright 2011 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Writes True if the argument is a directory."""
+
+import os.path
+import sys
+
+def main():
+  sys.stdout.write(_is_dir(sys.argv[1]))
+  return 0
+
+def _is_dir(dir_name):
+  return str(os.path.isdir(dir_name))
+
+def DoMain(args):
+  """Hook to be called from gyp without starting a separate python
+  interpreter."""
+  return _is_dir(args[0])
+
+if __name__ == '__main__':
+  sys.exit(main())
--- a/build/dotfile_settings.gni
+++ b/build/dotfile_settings.gni
@@ -24,6 +24,7 @@ build_dotfile_settings = {
     "//build/config/mac/mac_sdk.gni",
     "//build/config/mac/rules.gni",
     "//build/config/posix/BUILD.gn",
+    "//build/config/sysroot.gni",
     "//build/config/win/BUILD.gn",
     "//build/config/win/visual_studio_version.gni",
     "//build/rust/analyze.gni",
--- a/build/toolchain/siso.gni
+++ b/build/toolchain/siso.gni
@@ -7,11 +7,6 @@ import("//build/config/gclient_args.gni"
 use_siso_default = false
 
 _is_google_corp_machine = false
-if (path_exists("/usr/bin/gcert") ||  # linux
-    path_exists("/usr/local/bin/gcert") ||  # mac
-    path_exists("/c:/gnubby/bin/gcert.exe")) {
-  _is_google_corp_machine = true
-}
 
 _is_bot = false
 if (getenv("SWARMING_HEADLESS") != "" || getenv("SWARMING_BOT_ID") != "" ||
@@ -19,13 +14,6 @@ if (getenv("SWARMING_HEADLESS") != "" ||
   _is_bot = true
 }
 
-_is_ninja_used = path_exists(rebase_path(".ninja_deps", root_build_dir))
-
-if (path_exists("//build/config/siso/.sisoenv") &&
-    defined(build_with_chromium) && build_with_chromium &&
-    (_is_google_corp_machine || _is_bot) && !_is_ninja_used) {
-  use_siso_default = true
-}
 
 declare_args() {
   # Placeholder to allow having use_siso in args.gn file.