Revert of [iOS] Defaults to use system libjpeg 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")
 }
 
 declare_args() {
   # Uses system libjpeg. If true, overrides use_libjpeg_turbo.
-  use_system_libjpeg = is_ios
+  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") {
-  libs = [ "jpeg" ]
   defines = [ "USE_SYSTEM_LIBJPEG" ]
 }
 
 config("libjpeg_turbo_config") {
   defines = [ "USE_LIBJPEG_TURBO=1" ]
 }
 
 # This is a meta target that forwards to the system's libjpeg,
 # third_party/libjpeg, or third_party/libjpeg_turbo depending on the build args
 # declared in this file.
 group("jpeg") {
   if (use_system_libjpeg) {
+    libs = [ "jpeg" ]
     public_configs = [ ":system_libjpeg_config" ]
   } else if (use_libjpeg_turbo) {
     public_deps = [
       "//third_party/libjpeg_turbo:libjpeg",
     ]
     public_configs = [ ":libjpeg_turbo_config" ]
   } else {
     public_deps = [
       "//third_party/libjpeg:libjpeg",
     ]
   }
 }