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

ios/chrome/browser/ui/context_menu/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("context_menu") {
+  sources = [
+    "context_menu_coordinator.h",
+    "context_menu_coordinator.mm",
+  ]
+  deps = [
+    "//base",
+    "//ios/chrome/browser:browser_no_public_deps",
+    "//ios/chrome/browser/ui/alert_coordinator",
+    "//ios/web",
+    "//ui/base",
+    "//ui/strings",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "context_menu_coordinator_unittest.mm",
+  ]
+  deps = [
+    ":context_menu",
+    "//base",
+    "//ios/web",
+    "//testing/gtest",
+  ]
+}