headless: Add a small C++ example application

headless/app/headless_shell.h

Index: headless/app/headless_shell.h
diff --git a/headless/app/headless_shell.h b/headless/app/headless_shell.h
index 70c5936d81815f080485d9fe5cee18b308da89b3..4b0e83f6cbf3e5dd9f19b495af98d69e60599296 100644
--- a/headless/app/headless_shell.h
+++ b/headless/app/headless_shell.h
@@ -19,9 +19,9 @@ namespace headless {
 
 // An application which implements a simple headless browser.
 class HeadlessShell : public HeadlessWebContents::Observer,
-                      emulation::ExperimentalObserver,
-                      inspector::ExperimentalObserver,
-                      page::ExperimentalObserver {
+                      public emulation::ExperimentalObserver,
+                      public inspector::ExperimentalObserver,
+                      public page::ExperimentalObserver {
  public:
   HeadlessShell();
   ~HeadlessShell() override;