Teach skydb about the --trace-startup flag

sky/tools/skydb

Index: sky/tools/skydb
diff --git a/sky/tools/skydb b/sky/tools/skydb
index 6160030e48ceee4b422c19374f2f80b7a8e80fce..f8757f2557f430964e5dfc112f152ca042926d3c 100755
--- a/sky/tools/skydb
+++ b/sky/tools/skydb
@@ -113,6 +113,9 @@ class SkyDebugger(object):
             shell_args.append(
                 '--args-for=mojo:window_manager %s' % self._url_from_args(args))
 
+        if args.trace_startup:
+            shell_args.append('--trace-startup')
+
         # Map all mojo: urls to http: urls using the --origin command.
         build_dir_url = SkyServer.url_for_path(
             remote_server_port,
@@ -566,6 +569,7 @@ class SkyDebugger(object):
             default=DEFAULT_URL)
         start_parser.add_argument('--show-command', action='store_true',
             help='Display the shell command and exit')
+        start_parser.add_argument('--trace-startup', action='store_true')
         start_parser.set_defaults(func=self.start_command)
 
         stop_parser = subparsers.add_parser('stop',