gn: Add asserts that libjpeg_turbo is not used on iOS.

third_party/BUILD.gn

 # Copyright 2014 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.
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
+assert(!is_ios, "This is not used on iOS, don't drag it in unintentionally")
+
 declare_args() {
   # Uses system libjpeg. If true, overrides use_libjpeg_turbo.
   use_system_libjpeg = false
 
   # Uses libjpeg_turbo as the jpeg implementation. Has no effect if
   # use_system_libjpeg is set.
   use_libjpeg_turbo = true
 }
 
 config("system_libjpeg_config") {
@@ skipping 15 matching lines @@
     public_deps = [
       "//third_party/libjpeg_turbo:libjpeg",
     ]
     public_configs = [ ":libjpeg_turbo_config" ]
   } else {
     public_deps = [
       "//third_party/libjpeg:libjpeg",
     ]
   }
 }