Reland of iOS: Use JSON for GN configuration instead of MB + remove symbols

Reason for revert:
MB is now updated to handle spaces in the buildername properly, something that
only affected our commit bots (not trybots).

Original issue's description:
> Revert of iOS: Use JSON for GN configuration instead of MB + remove symbols (patchset #4 id:80001 of https://codereview.webrtc.org/2688103002/ )
>
> Reason for revert:
> Something is different from trybots vs the commit bots, causing it to fail when running GN:
> https://build.chromium.org/p/client.webrtc/builders/iOS32%20Release/builds/10151
>
> Original issue's description:
> > iOS: Use JSON for GN configuration instead of MB + remove symbols.
> >
> > This aligns with how the ios recipe module is used in Chromium.
> > It should prevent breakages like one we had recently.
> >
> > It also means we're no longer setting symbol_level=1 explicitly.
> > The default is 0 (no symbols), which is now what's being used.
> >
> > Also move all the directories containing JSON files into
> > tools-webrtc/ios/bots to make it clearer (and more similar to
> > Chromium).
> >
> > BUG=webrtc:7140, webrtc:7161
> > NOTRY=True
> >
> > Review-Url: https://codereview.webrtc.org/2688103002
> > Cr-Commit-Position: refs/heads/master@{#16633}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/73f01de4edda30c419bf6daf8b91d2a0d28c06a9
>
> TBR=ehmaldonado@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7140, webrtc:7161
>
> Review-Url: https://codereview.webrtc.org/2694423002
> Cr-Commit-Position: refs/heads/master@{#16634}
> Committed: https://chromium.googlesource.com/external/webrtc/+/68ab366547b7494b69e28047b17ad2ebd64e9edd

TBR=ehmaldonado@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7140, webrtc:7161

Review-Url: https://codereview.webrtc.org/2697133002
Cr-Commit-Position: refs/heads/master@{#16636}
diff --git a/tools-webrtc/mb/mb_unittest.py b/tools-webrtc/mb/mb_unittest.py
index f5c5bc2..67e1d75 100755
--- a/tools-webrtc/mb/mb_unittest.py
+++ b/tools-webrtc/mb/mb_unittest.py
@@ -25,7 +25,7 @@
 
     # Override vars for test portability.
     if win32:
-      self.chromium_src_dir = 'c:\\fake_src'
+      self.src_dir = 'c:\\fake_src'
       self.default_config = 'c:\\fake_src\\tools-webrtc\\mb\\mb_config.pyl'
       self.default_isolate_map = ('c:\\fake_src\\testing\\buildbot\\'
                                   'gn_isolate_map.pyl')
@@ -33,7 +33,7 @@
       self.executable = 'c:\\python\\python.exe'
       self.sep = '\\'
     else:
-      self.chromium_src_dir = '/fake_src'
+      self.src_dir = '/fake_src'
       self.default_config = '/fake_src/tools-webrtc/mb/mb_config.pyl'
       self.default_isolate_map = '/fake_src/testing/buildbot/gn_isolate_map.pyl'
       self.executable = '/usr/bin/python'