Reland Enable hermetic builds for iOS.

build_overrides/build.gni

Index: build_overrides/build.gni
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index a7cd058756591b58e2556af74a8268457632364a..e88dfc98383306d45021f89ce5fedeedc2c75059 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -46,12 +46,8 @@ declare_args() {
 }
 
 if (use_system_xcode == "") {
-  if (target_os == "mac") {
-    _result =
-        exec_script("//build/mac/should_use_hermetic_xcode.py", [], "value")
-    use_system_xcode = _result == 0
-  }
-  if (target_os == "ios") {
-    use_system_xcode = true
-  }
+  _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
+                        [ target_os ],
+                        "value")
+  use_system_xcode = _result == 0
 }