[Webrtc] Reenable libc++ on gcc

tools_webrtc/mb/mb_config.pyl

 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # FOR DETAILS ON THIS FILE SEE THE MAIN COPY IN //tools/mb/mb_config.pyl.
 # This is configuration for standalone WebRTC bots. It is used to keep the bot
@@ skipping 182 matching lines @@
     }
   },
 
   # This is the list of configs that you can pass to mb; each config
   # represents a particular combination of gn args that we must support.
   # A given config *may* be platform-specific but is not necessarily so (i.e.,
   # we might have mac, win, and linux bots all using the 'release_bot' config).
   'configs': {
     # Linux, Mac and Windows
     'gcc_release_bot_x64': [
-      'gn', 'gcc', 'release_bot', 'x64', 'no_rtc_tests', 'use_default_libcxx'
+      'gn', 'gcc', 'release_bot', 'x64', 'no_rtc_tests'
     ],
     # TOOD(kjellander): Restore Goma for this when crbug.com/726706 is fixed.
     'debug_bot_arm': [
       'gn', 'openh264', 'debug', 'arm'
     ],
     'release_bot_arm': [
       'gn', 'openh264_release_bot', 'arm'
     ],
     'debug_bot_arm64': [
       'gn', 'openh264_debug_bot', 'arm64'
@@ skipping 258 matching lines @@
 
     'x86': {
       'gn_args': 'target_cpu="x86"',
     },
 
     'use_default_libcxx': {
       'gn_args': 'use_custom_libcxx=false',
     }
   },
 }