autoninja.py: Enable offline mode for Siso when use_remoteexec not set

Tested with:

$ autoninja -C out/fastbuild-siso-local base
offline mode
 2.48s loading build.ninja
pre:0 local:176 remote:0 5.7/s fallback:0
[...]
5m40.86s Build Succeeded: 1641 steps - 4.81/s

$ autoninja -C out/fastbuild-siso-reclient base
Running gcloud auth print-access-token --trace-token=interactiveauth
Proxy started successfully.
124.260 s to start reproxy
 5.32s loading build.ninja
15.93s Regenerating ninja files
 2.75s update filegroups
 4.84s loading build.ninja
pre:0 local:1 remote:0 30.2/s cache:100.00% fallback:0
[...]
2m05.55s Build Succeeded: 2560 steps - 20.39/s

R=jwata, ukai

Change-Id: Ib976b874796a20dca55005986ee6f7ee3c942245
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4997466
Commit-Queue: Philipp Wollermann <philwo@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>

autoninja.py

@@ -183,7 +183,7 @@ def main(args):
                 return 1
             if use_remoteexec:
                 return autosiso.main(['autosiso'] + input_args[1:])
-            return siso.main(['siso', 'ninja'] + input_args[1:])
+            return siso.main(['siso', 'ninja', '--offline'] + input_args[1:])
 
         if os.path.exists(siso_marker):
             print('Run gn clean before switching from siso to ninja in %s' %