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

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 573 matching lines @@
     set_default_toolchain("//build/toolchain/linux:$current_cpu")
   }
   if (is_chromeos && cros_use_custom_toolchain) {
     set_default_toolchain("//build/toolchain/cros:target")
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang_x64"
   set_default_toolchain(host_toolchain)
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:clang_x64"
-  set_default_toolchain("//build/toolchain/mac:clang_$current_cpu")
+  set_default_toolchain("//build/toolchain/mac:ios_clang_arm")
 } else if (is_nacl) {
   # TODO(GYP): This will need to change when we get NaCl working
   # on multiple platforms, but this whole block of code (how we define
   # host_toolchain) needs to be reworked regardless to key off of host_os
   # and host_cpu rather than the is_* variables.
   host_toolchain = "//build/toolchain/linux:clang_x64"
 }
 
 # ==============================================================================
 # COMPONENT SETUP
@@ skipping 189 matching lines @@
       }
       if (defined(invoker.testonly)) {
         testonly = invoker.testonly
       }
       if (defined(invoker.visibility)) {
         visibility = invoker.visibility
       }
     }
   }
 }