autoninja: Enable Siso by default for external Chromium developers

This CL changes the default build system for Chromium builds by external
developers from Ninja to Siso.
See also https://crrev.com/c/6638830 for more info.

Bug: 424659245, 409223168
Change-Id: I382746cab2e8b40db5f555711e965f814d089987
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6636690
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>

autoninja.py

@@ -189,11 +189,6 @@ def _get_use_siso_default(output_dir):
     if not os.path.exists(sisoenv_path):
         return False
 
-    # Use Siso by default on Googlers on corp machine for now.
-    # TODO: crbug.com/409223168 - Enable Siso by default for external devs.
-    if not _is_google_corp_machine():
-        return False
-
     # Check the project wide default in `.gn`.
     dot_gn = os.path.join(root_dir, ".gn")
     if os.path.exists(dot_gn):