[ios] Add free function to (de)serialise WebStateList.

ios/shared/chrome/browser/tabs/BUILD.gn

Index: ios/shared/chrome/browser/tabs/BUILD.gn
diff --git a/ios/shared/chrome/browser/tabs/BUILD.gn b/ios/shared/chrome/browser/tabs/BUILD.gn
index 484ea14f7533979e9ce047d552a49279eef48d0e..42b01ee233e4e8df8beac9d3631ef74dc64abbcf 100644
--- a/ios/shared/chrome/browser/tabs/BUILD.gn
+++ b/ios/shared/chrome/browser/tabs/BUILD.gn
@@ -17,6 +17,8 @@ source_set("tabs") {
     "web_state_list_observer_bridge.mm",
     "web_state_list_order_controller.h",
     "web_state_list_order_controller.mm",
+    "web_state_list_serialization.h",
+    "web_state_list_serialization.mm",
     "web_state_opener.h",
     "web_state_opener.mm",
   ]
@@ -47,6 +49,7 @@ source_set("unit_tests") {
   sources = [
     "web_state_list_fast_enumeration_helper_unittest.mm",
     "web_state_list_order_controller_unittest.mm",
+    "web_state_list_serialization_unittest.mm",
     "web_state_list_unittest.mm",
     "web_state_opener_unittest.mm",
   ]
@@ -54,6 +57,7 @@ source_set("unit_tests") {
     ":tabs",
     ":test_support",
     "//base",
+    "//ios/web",
     "//ios/web:test_support",
     "//net",
     "//testing/gtest",