Revert of [headless] Use individual aura::WindowTreeHosts per WebContents.

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index 8f67953f92c9d9f8b386b5a8e5cf5d7a6264823d..d54ecfd77eacd65e9f32707b0568fee1fa44d4cf 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -17,6 +17,16 @@
 #include "headless/lib/browser/headless_devtools_manager_delegate.h"
 #include "headless/lib/browser/headless_web_contents_impl.h"
 #include "headless/public/headless_export.h"
+
+#if defined(USE_AURA)
+#include "headless/lib/browser/headless_window_tree_host.h"
+
+namespace aura {
+namespace client {
+class FocusClient;
+}
+}
+#endif
 
 namespace headless {
 
@@ -71,9 +81,17 @@
   void PlatformInitialize();
   void PlatformCreateWindow();
   void PlatformInitializeWebContents(const gfx::Size& initial_size,
-                                     HeadlessWebContentsImpl* web_contents);
+                                     content::WebContents* web_contents);
 
  protected:
+#if defined(USE_AURA)
+  // TODO(eseckler): Currently one window and one window_tree_host
+  // is used for all web contents. We should probably use one
+  // window per web contents, but additional investigation is needed.
+  std::unique_ptr<HeadlessWindowTreeHost> window_tree_host_;
+  std::unique_ptr<aura::client::FocusClient> focus_client_;
+#endif
+
   base::Callback<void(HeadlessBrowser*)> on_start_callback_;
   HeadlessBrowser::Options options_;
   HeadlessBrowserMainParts* browser_main_parts_;  // Not owned.