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

headless/lib/browser/headless_window_tree_host.h

Index: headless/lib/browser/headless_window_tree_host.h
diff --git a/headless/lib/browser/headless_window_tree_host.h b/headless/lib/browser/headless_window_tree_host.h
index 097435acca1ea53c86217284196ada6d3c710703..0aadfc87d18c37737ea8d0e198d1611373714ce1 100644
--- a/headless/lib/browser/headless_window_tree_host.h
+++ b/headless/lib/browser/headless_window_tree_host.h
@@ -5,21 +5,13 @@
 #ifndef HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_
 
-#if defined(USE_AURA)
-
 #include <memory>
 
 #include "base/macros.h"
+#include "headless/lib/browser/headless_window_parenting_client.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/rect.h"
-
-namespace aura {
-namespace client {
-class FocusClient;
-class WindowParentingClient;
-}
-}
 
 namespace headless {
 
@@ -52,7 +44,6 @@
 
  private:
   gfx::Rect bounds_;
-  std::unique_ptr<aura::client::FocusClient> focus_client_;
   std::unique_ptr<aura::client::WindowParentingClient> window_parenting_client_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessWindowTreeHost);
@@ -60,8 +51,4 @@
 
 }  // namespace headless
 
-#else   // defined(USE_AURA)
-class HeadlessWindowTreeHost {};
-#endif  // defined(USE_AURA)
-
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_