headless: Add a small C++ example application

headless/BUILD.gn

Index: headless/BUILD.gn
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index 1927eb256b9326fa95cc13799524bb02fa4b52b9..151af1bdd906d12f0565551b591ebe057d69adfd 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -323,6 +323,7 @@ group("headless_tests") {
 
   deps = [
     ":headless_browsertests",
+    ":headless_example",
     ":headless_unittests",
   ]
 }
@@ -459,3 +460,13 @@ process_version("version_header") {
   ]
   output = "$target_gen_dir/public/version.h"
 }
+
+executable("headless_example") {
+  sources = [
+    "app/headless_example.cc",
+  ]
+
+  deps = [
+    "//headless:headless_shell_lib",
+  ]
+}