headless: Add a dedicated child process entrypoint

headless/app/headless_shell.cc

Index: headless/app/headless_shell.cc
diff --git a/headless/app/headless_shell.cc b/headless/app/headless_shell.cc
index f596ae2a7e707a4d7ea8852d467bedbb9cc4b097..f9e8e3f45d4ecc6292458fe1f135f797bb7b7c0a 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -285,6 +285,7 @@ class HeadlessShell : public HeadlessWebContents::Observer, page::Observer {
 };
 
 int main(int argc, const char** argv) {
+  headless::RunChildProcessIfNeeded(argc, argv);
   HeadlessShell shell;
   HeadlessBrowser::Options::Builder builder(argc, argv);