Add ios_use_simulator variable used by Skia build scripts (#846)

build/config/ios/ios_sdk.gni

diff --git a/build/config/ios/ios_sdk.gni b/build/config/ios/ios_sdk.gni
index e2f79fd..c70dadc 100644
--- a/build/config/ios/ios_sdk.gni
+++ b/build/config/ios/ios_sdk.gni
@@ -16,6 +16,9 @@
   # existing GYP build).
   use_ios_simulator = true
 
+  # Alias for use_ios_simulator used by Skia.
+  ios_use_simulator = true
+
   # Version of iOS that we're targeting.
   ios_deployment_target = "12.0"