[ios] Adds code to support the iOS Handoff feature.

components/handoff/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.
 
 source_set("handoff") {
   if (is_mac || is_ios) {
     sources = [
+      "handoff_manager.h",
+      "handoff_manager.mm",
       "handoff_utility.h",
       "handoff_utility.mm",
     ]
-  }
-  if (is_mac) {
-    sources += [
-      "handoff_manager.h",
-      "handoff_manager.mm",
-    ]
     deps = [
       "//base",
       "//net",
       "//url",
     ]
   }
+  if (is_ios) {
+    sources += [
+      "pref_names_ios.cc",
+      "pref_names_ios.h",
+    ]
+    deps += [ "//components/pref_registry" ]
+  }
 }