Add a gn implementation for //ios/public/provider/web target.

ios/public/provider/web/BUILD.gn

Index: ios/public/provider/web/BUILD.gn
diff --git a/ios/public/provider/web/BUILD.gn b/ios/public/provider/web/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..2df8a62c3dcde749d1f5e83b65e8f912295df5e5
--- /dev/null
+++ b/ios/public/provider/web/BUILD.gn
@@ -0,0 +1,21 @@
+# 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("web") {
+  sources = [
+    "web_controller_provider.h",
+    "web_controller_provider.mm",
+    "web_ui_ios.h",
+    "web_ui_ios_controller.cc",
+    "web_ui_ios_controller.h",
+    "web_ui_ios_controller_factory.h",
+    "web_ui_ios_message_handler.cc",
+    "web_ui_ios_message_handler.h",
+  ]
+
+  deps = [
+    "//base",
+    "//ios/web",
+  ]
+}