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

headless/lib/browser/headless_window_tree_host.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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 {
 
 class HeadlessWindowTreeHost : public aura::WindowTreeHost,
                                public ui::PlatformEventDispatcher {
  public:
   explicit HeadlessWindowTreeHost(const gfx::Rect& bounds);
   ~HeadlessWindowTreeHost() override;
 
   void SetParentWindow(gfx::NativeWindow window);
 
@@ skipping 11 matching lines @@
   gfx::Point GetLocationOnScreenInPixels() const override;
   void SetCapture() override;
   void ReleaseCapture() override;
   void SetCursorNative(gfx::NativeCursor cursor_type) override;
   void MoveCursorToScreenLocationInPixels(const gfx::Point& location) override;
   void OnCursorVisibilityChangedNative(bool show) override;
   gfx::ICCProfile GetICCProfileForCurrentDisplay() override;
 
  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);
 };
 
 }  // namespace headless
 
-#else   // defined(USE_AURA)
-class HeadlessWindowTreeHost {};
-#endif  // defined(USE_AURA)
-
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_