[iOS] Defaults to use system libjpeg on iOS.

third_party/BUILD.gn

Index: third_party/BUILD.gn
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index cf62b83fca29975a1817d3791b0110c2cf9250a0..6d97aafdbce05c75ea1a3ed68552d77ba115b94e 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -8,7 +8,7 @@ if (is_android) {
 
 declare_args() {
   # Uses system libjpeg. If true, overrides use_libjpeg_turbo.
-  use_system_libjpeg = false
+  use_system_libjpeg = is_ios
 
   # Uses libjpeg_turbo as the jpeg implementation. Has no effect if
   # use_system_libjpeg is set.
@@ -16,6 +16,7 @@ declare_args() {
 }
 
 config("system_libjpeg_config") {
+  libs = [ "jpeg" ]
   defines = [ "USE_SYSTEM_LIBJPEG" ]
 }
 
@@ -28,7 +29,6 @@ config("libjpeg_turbo_config") {
 # declared in this file.
 group("jpeg") {
   if (use_system_libjpeg) {
-    libs = [ "jpeg" ]
     public_configs = [ ":system_libjpeg_config" ]
   } else if (use_libjpeg_turbo) {
     public_deps = [