mb: Implement 'quiet' flag in mb lookup

This is a partial port of https://chromium-review.googlesource.com/1576022
Needed because recipes assume the flag exists.

Example error (unnoticed until now because it's silently ignored):
https://logs.chromium.org/logs/webrtc/buildbucket/cr-buildbucket.appspot.com/8911948550157608800/+/steps/lookup_GN_args/0/stdout

Bug: chromium:955062
Change-Id: Id3707903dd5747c13bba05df76f799fb523a6faa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/139248
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28123}

tools_webrtc/mb/mb.py

@@ -157,6 +157,9 @@ class MetaBuildWrapper(object):
                             help='look up the command for a given config or '
                                  'builder')
     AddCommonOptions(subp)
+    subp.add_argument('--quiet', default=False, action='store_true',
+                      help='Print out just the arguments, '
+                           'do not emulate the output of the gen subcommand.')
     subp.set_defaults(func=self.CmdLookup)
 
     subp = subps.add_parser(
@@ -286,12 +289,15 @@ class MetaBuildWrapper(object):
 
   def CmdLookup(self):
     vals = self.Lookup()
-    cmd = self.GNCmd('gen', '_path_')
     gn_args = self.GNArgs(vals)
-    self.Print('\nWriting """\\\n%s""" to _path_/args.gn.\n' % gn_args)
-    env = None
+    if self.args.quiet:
+      self.Print(gn_args, end='')
+    else:
+      cmd = self.GNCmd('gen', '_path_')
+      self.Print('\nWriting """\\\n%s""" to _path_/args.gn.\n' % gn_args)
+      env = None
 
-    self.PrintCmd(cmd, env)
+      self.PrintCmd(cmd, env)
     return 0
 
   def CmdRun(self):