gn: Default is_clang to true on 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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 143 matching lines @@
   is_win = false
 } else if (os == "ios") {
   is_android = false
   is_chromeos = false
   is_ios = true
   is_linux = false
   is_mac = false
   is_nacl = false
   is_posix = true
   is_win = false
+  if (!is_clang) {
+    # Always use clang on iOS when using ninja
+    # (which is always true when using GN).
+    is_clang = true
+  }
 } else if (os == "linux") {
   is_android = false
   is_chromeos = false
   is_ios = false
   is_linux = true
   is_mac = false
   is_nacl = false
   is_posix = true
   is_win = false
 }
@@ skipping 261 matching lines @@
 } else if (is_android) {
   host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   set_default_toolchain("//build/toolchain/android:$cpu_arch")
 } else if (is_linux) {
   host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   set_default_toolchain("//build/toolchain/linux:$cpu_arch")
 } else if (is_mac || is_ios) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }