[iOS] Define build/config/ios:ios_dynamic_flags for default linker flags.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 9e352eb4d5c3c139320c94e42ef8d9f7574855e9..6a3133d9ecc6bcd02848a45b4a1803ce15627bf4 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -389,6 +389,8 @@ config("executable_config") {
       "//build/config/mac:mac_dynamic_flags",
       "//build/config/mac:mac_executable_flags",
     ]
+  } else if (is_ios) {
+    configs += [ "//build/config/ios:ios_dynamic_flags" ]
   } else if (is_linux || is_android) {
     configs += [ "//build/config/gcc:executable_ldconfig" ]
     if (is_android) {
@@ -418,6 +420,8 @@ config("shared_library_config") {
     configs += _windows_linker_configs
   } else if (is_mac) {
     configs += [ "//build/config/mac:mac_dynamic_flags" ]
+  } else if (is_ios) {
+    configs += [ "//build/config/ios:ios_dynamic_flags" ]
   } else if (is_chromecast) {
     configs += [ "//build/config/chromecast:shared_library_config" ]
   }