Update build.gni to match Chromium

The last change was March 2019
diff --git a/build.gni b/build.gni
index 7b632bd..e8471bf 100644
--- a/build.gni
+++ b/build.gni
@@ -2,19 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# Several dependencies of Chrome require a different min sdk when built
-# as stand-alone projects. If this is ever not the case, these variables
-# can be removed.
-# See https://bugs.chromium.org/p/webrtc/issues/detail?id=5453.
-mac_sdk_min_build_override = "10.10"
-
-# Variable that can be used to support multiple build scenarios, like having
-# Chromium specific targets in a client project's GN file etc.
-build_with_chromium = true
+import("//build/config/gclient_args.gni")
 
 # Uncomment these to specify a different NDK location and version in
 # non-Chromium builds.
-# default_android_ndk_root = "//third_party/android_tools/ndk"
+# default_android_ndk_root = "//third_party/android_ndk"
 # default_android_ndk_version = "r10e"
 
 # Some non-Chromium builds don't support building java targets.
@@ -50,13 +42,21 @@
   # This does not affect the compiler. When this variable is false, targets will
   # instead use a hermetic install of Xcode. [The hermetic install can be
   # obtained with gclient sync after setting the environment variable
-  # FORCE_MAC_TOOLCHAIN].
+  # FORCE_MAC_TOOLCHAIN for target_os mac only].
   use_system_xcode = ""
 }
 
-if (use_system_xcode == "") {
+if (host_os == "mac" && use_system_xcode == "") {
   _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
                         [ target_os ],
                         "value")
-  use_system_xcode = _result == 0
+
+  assert(_result != 2,
+         "Do not allow building targets with the default" +
+             "hermetic toolchain if the minimum OS version is not met.")
+  assert(_result != 3,
+         "iOS does not support building with a hermetic toolchain. " +
+             "Please install Xcode.")
+
+  use_system_xcode = _result != 1
 }