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}

tools-webrtc/mb/mb_unittest.py

@@ -25,7 +25,7 @@ class FakeMBW(mb.MetaBuildWrapper):
 
     # 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 @@ class FakeMBW(mb.MetaBuildWrapper):
       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'