[ios] Add a delegate to WebStateList class.

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 9dfe5783a74f445fdde4956270eec64468d4a43a..af0ef7244e16996a7b4a1842fb4c0b2f4723e53d 100644
--- a/ios/shared/chrome/browser/tabs/BUILD.gn
+++ b/ios/shared/chrome/browser/tabs/BUILD.gn
@@ -6,6 +6,7 @@ source_set("tabs") {
   sources = [
     "web_state_list.h",
     "web_state_list.mm",
+    "web_state_list_delegate.h",
     "web_state_list_fast_enumeration_helper.h",
     "web_state_list_fast_enumeration_helper.mm",
     "web_state_list_metrics_observer.h",
@@ -26,6 +27,19 @@ source_set("tabs") {
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
+source_set("test_support") {
+  testonly = true
+  sources = [
+    "fake_web_state_list_delegate.h",
+    "fake_web_state_list_delegate.mm",
+  ]
+  deps = [
+    ":tabs",
+    "//base",
+  ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [
@@ -35,6 +49,7 @@ source_set("unit_tests") {
   ]
   deps = [
     ":tabs",
+    ":test_support",
     "//base",
     "//ios/web:test_support",
     "//net",