headless: Add a small C++ example application

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 06940881c3f58f14d53e8fa4f5461c57b2398c8a..04300eec603be7f07906207771a416895ff76ce7 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -261,14 +261,14 @@ void HeadlessShell::OnPageReady() {
 
 void HeadlessShell::FetchDom() {
   devtools_client_->GetRuntime()->Evaluate(
-      "document.body.innerHTML",
+      "document.body.outerHTML",
       base::Bind(&HeadlessShell::OnDomFetched, weak_factory_.GetWeakPtr()));
 }
 
 void HeadlessShell::OnDomFetched(
     std::unique_ptr<runtime::EvaluateResult> result) {
   if (result->HasExceptionDetails()) {
-    LOG(ERROR) << "Failed to evaluate document.body.innerHTML: "
+    LOG(ERROR) << "Failed to evaluate document.body.outerHTML: "
                << result->GetExceptionDetails()->GetText();
   } else {
     std::string dom;