Split //ios/chrome/browser in multiple targets.

ios/chrome/browser/itunes_links/BUILD.gn

+# Copyright 2016 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("itunes_links") {
+  sources = [
+    "itunes_links_observer.h",
+    "itunes_links_observer.mm",
+  ]
+  deps = [
+    "//base",
+    "//ios/chrome/browser:browser_no_public_deps",
+    "//ios/web",
+    "//url",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "itunes_links_observer_unittest.mm",
+  ]
+  deps = [
+    ":itunes_links",
+    "//base",
+    "//ios/chrome/browser",
+    "//ios/web:test_support",
+    "//testing/gtest",
+    "//third_party/ocmock",
+    "//url",
+  ]
+}