Add Desktop iOS promotion logging to chrome ios app.

ios/chrome/browser/desktop_promotion/BUILD.gn

Index: ios/chrome/browser/desktop_promotion/BUILD.gn
diff --git a/ios/chrome/browser/desktop_promotion/BUILD.gn b/ios/chrome/browser/desktop_promotion/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..7c27d0f84094b2a7d04a93b41232a242cef61a7d
--- /dev/null
+++ b/ios/chrome/browser/desktop_promotion/BUILD.gn
@@ -0,0 +1,27 @@
+# Copyright 2017 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("desktop_promotion") {
+  sources = [
+    "desktop_promotion_sync_observer.h",
+    "desktop_promotion_sync_observer.mm",
+    "desktop_promotion_sync_service.cc",
+    "desktop_promotion_sync_service.h",
+    "desktop_promotion_sync_service_factory.cc",
+    "desktop_promotion_sync_service_factory.h",
+    "histogram_macros.h",
+  ]
+  deps = [
+    "//base",
+    "//components/browser_sync",
+    "//components/keyed_service/ios",
+    "//components/metrics",
+    "//components/pref_registry",
+    "//components/prefs",
+    "//ios/chrome/browser",
+    "//ios/chrome/browser/browser_state",
+    "//ios/chrome/browser/sync/",
+    "//ios/chrome/common",
+  ]
+}