Upstream the iOS web_shell and supporting code

ios/web/public/app/web_main_parts.mm

Index: ios/web/public/app/web_main_parts.mm
diff --git a/ios/build/packaging/dummy_main.mm b/ios/web/public/app/web_main_parts.mm
similarity index 59%
copy from ios/build/packaging/dummy_main.mm
copy to ios/web/public/app/web_main_parts.mm
index 20dd1ba9897fabfbc3bd480c374e0526dbdfe90d..541cc36243b99edd84222c0a88d8d224906adea3 100644
--- a/ios/build/packaging/dummy_main.mm
+++ b/ios/web/public/app/web_main_parts.mm
@@ -2,7 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Provides a main() function for an empty dummy app.
-int main(int argc, char *argv[]) {
+#include "ios/web/public/app/web_main_parts.h"
+
+namespace web {
+
+int WebMainParts::PreCreateThreads() {
   return 0;
 }
+
+}  // namespace web