autoninja: report if 'use_remoteexec=true' is missing.

Change-Id: Id5c61b31dd46e77ff1308746e388b2347919a0a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6082931
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>

autoninja.py

@@ -336,6 +336,13 @@ def _main_inner(input_args, build_id, should_collect_logs=False):
                         ] + input_args[1:],
                         should_collect_logs)
                 return siso.main(["siso", "ninja"] + input_args[1:])
+            if not project:
+                project = _siso_rbe_project()
+            if not t_specified and project and not offline:
+                print(
+                    'Missing "use_remoteexec=true". No remote execution',
+                    file=sys.stderr,
+                )
             return siso.main(["siso", "ninja", "--offline"] + input_args[1:])
 
         if os.path.exists(siso_marker):