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

ios/web/public/app/BUILD.gn

Index: ios/web/public/app/BUILD.gn
diff --git a/mojo/test/BUILD.gn b/ios/web/public/app/BUILD.gn
similarity index 63%
copy from mojo/test/BUILD.gn
copy to ios/web/public/app/BUILD.gn
index 05f54a4036b29005638e2fd83324dbf2cd493c77..b2e21aa84dc974a306e49f820526030c9f6ee8ba 100644
--- a/mojo/test/BUILD.gn
+++ b/ios/web/public/app/BUILD.gn
@@ -2,14 +2,18 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("test_support") {
+source_set("app") {
   sources = [
-    "test_utils.h",
-    "test_utils_posix.cc",
-    "test_utils_win.cc",
+    "web_main.h",
+    "web_main_delegate.h",
+    "web_main_parts.h",
   ]
 
   public_deps = [
     "//base",
   ]
+
+  deps = [
+    "//ios/web/app",
+  ]
 }