Add gn implementation of //ios/web/shell:ios_web_shell.

ios/web/app/BUILD.gn

+# Copyright 2015 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("app") {
+  visibility = [ "//ios/web/public/app" ]
+
+  sources = [
+    "web_main.mm",
+    "web_main_loop.h",
+    "web_main_loop.mm",
+    "web_main_runner.h",
+    "web_main_runner.mm",
+  ]
+
+  deps = [
+    "//base",
+    "//base:i18n",
+    "//crypto",
+    "//ios/web",
+    "//net",
+    "//ui/base",
+    "//ui/gfx",
+    "//ui/gfx/geometry",
+  ]
+
+  libs = [
+    "Foundation.framework",
+    "UIKit.framework",
+  ]
+}