[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 0aadfc87d18c37737ea8d0e198d1611373714ce1..097435acca1ea53c86217284196ada6d3c710703 100644
--- a/headless/lib/browser/headless_window_tree_host.h
+++ b/headless/lib/browser/headless_window_tree_host.h
@@ -5,14 +5,22 @@
 #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,
@@ -44,6 +52,7 @@ class HeadlessWindowTreeHost : public aura::WindowTreeHost,
 
  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);
@@ -51,4 +60,8 @@ class HeadlessWindowTreeHost : public aura::WindowTreeHost,
 
 }  // namespace headless
 
+#else   // defined(USE_AURA)
+class HeadlessWindowTreeHost {};
+#endif  // defined(USE_AURA)
+
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_