Reland of Add tools/mb to setup_links.py (patchset #1 id:1 of https://codereview.webrtc.org/1691723003/ )

Reason for revert:
Actually, we're going to need this soon, so I'm going to add it back again. We'll use it as part of the future GN migration.

Original issue's description:
> Revert of Add tools/mb to setup_links.py (patchset #1 id:1 of https://codereview.webrtc.org/1692543002/ )
>
> Reason for revert:
> Doesn't solve the problem and MB is actually hardcoded to use gyp_chromium when running in GYP mode, so it's better to clean this up for now.
>
> Original issue's description:
> > Add tools/mb to setup_links.py
> >
> > I believe this is needed in order to setup the iOS simulator bots
> > using the ios recipe module. We'll also most likely use MB in the future
> > when moving over from GYP to GN (since that's the sole purpose of the tool:
> > to make such a migration easier).
> >
> > BUG=chromium:498746
> > NOTRY=True
> > TBR=phoglund@webrtc.org
> >
> > Committed: https://crrev.com/68da769ab9579cac98fe4da7fb5da6dbb3842216
> > Cr-Commit-Position: refs/heads/master@{#11575}
>
> TBR=phoglund@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=chromium:498746
>
> Committed: https://crrev.com/67ca0e16501d5c9179d3003a38a7cefc7cd3e259
> Cr-Commit-Position: refs/heads/master@{#11577}

TBR=phoglund@webrtc.org
NOTRY=True

Review URL: https://codereview.webrtc.org/1730113002

Cr-Commit-Position: refs/heads/master@{#11732}

setup_links.py

@@ -69,6 +69,7 @@ DIRECTORIES = [
   'tools/generate_stubs',
   'tools/gn',
   'tools/gyp',
+  'tools/mb',
   'tools/memory',
   'tools/protoc_wrapper',
   'tools/python',