patch from chinmaygarde@ to make progress on mac, ios.

build/config/ios/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sysroot.gni")
 import("//build/config/ios/ios_sdk.gni")
 
 config("sdk") {
   common_flags = [
     "-isysroot",
     sysroot,
+
+    "-stdlib=libc++",
   ]
 
   if (use_ios_simulator) {
     common_flags += [ "-mios-simulator-version-min=$ios_deployment_target" ]
   } else {
     common_flags += [ "-miphoneos-version-min=$ios_deployment_target" ]
   }
 
   cflags = common_flags
   ldflags = common_flags
 }