Work on GN iOS build.

build/config/ios/ios_sdk.gni

Index: build/config/ios/ios_sdk.gni
diff --git a/build/config/ios/ios_sdk.gni b/build/config/ios/ios_sdk.gni
index f941f3ce09938d91bbd9a0759aa2e7b0f9ed7f85..7acbaaa17d78c5e5a31372172261707e53fcb422 100644
--- a/build/config/ios/ios_sdk.gni
+++ b/build/config/ios/ios_sdk.gni
@@ -2,9 +2,31 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# TODO(brettw) support "iphoneos" in addition to the simulator. May also need
-# support for common.gypi's "ios_sdk" variable (seems to be a version number)
-# and ios_sdk_path (argument that overrides the one returned below).
-ios_sdk_result =
-  exec_script("ios_sdk.py", [ "iphonesimulator" ], "list lines")
-ios_sdk_path = ios_sdk_result[0]
+declare_args() {
+  # SDK path to use. When empty this will use the default SDK based on the
+  # value of use_ios_simulator.
+  ios_sdk_path = ""
+
+  # Set to true when targeting a simulator build on iOS. False means that the
+  # target is for running on the device.
+  use_ios_simulator = true
+
+  ios_deployment_target = "6.0"
+}
+
+if (!is_gyp_xcode_generator) {
+  # The Ninja build currently only targets the simulator.
+  assert(use_ios_simulator, "You can't do an iOS device build using Ninja yet.")
+}
+
+if (ios_sdk_path == "") {
+  # Compute default target.
+  if (use_ios_simulator) {
+    _ios_sdk_to_query = "iphonesimulator"
+  } else {
+    _ios_sdk_to_query = "iphoneos"
+  }
+  _ios_sdk_result =
+    exec_script("ios_sdk.py", [ _ios_sdk_to_query ], "list lines")
+  ios_sdk_path = _ios_sdk_result[0]
+}