Teach skydb about the --trace-startup flag

This flag to mojo_shell is gaining the ability to produce tracing info
starting very early in process startup. This allows the skydb 'start'
command to pass this command through to mojo_shell.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1065723003

engine/src/flutter/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',