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

headless/lib/browser/headless_window_tree_host.cc

 // 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.
 
 #include "headless/lib/browser/headless_window_tree_host.h"
+
+#include "headless/lib/browser/headless_focus_client.h"
+#include "headless/lib/browser/headless_window_parenting_client.h"
 #include "ui/aura/window.h"
-
 #include "ui/gfx/icc_profile.h"
 
 namespace headless {
 
 HeadlessWindowTreeHost::HeadlessWindowTreeHost(const gfx::Rect& bounds)
     : bounds_(bounds) {
   CreateCompositor();
   OnAcceleratedWidgetAvailable();
+
+  focus_client_.reset(new HeadlessFocusClient());
+  aura::client::SetFocusClient(window(), focus_client_.get());
 }
 
 HeadlessWindowTreeHost::~HeadlessWindowTreeHost() {
   window_parenting_client_.reset();
   DestroyCompositor();
   DestroyDispatcher();
 }
 
 void HeadlessWindowTreeHost::SetParentWindow(gfx::NativeWindow window) {
   window_parenting_client_.reset(new HeadlessWindowParentingClient(window));
@@ skipping 46 matching lines @@
 void HeadlessWindowTreeHost::MoveCursorToScreenLocationInPixels(
     const gfx::Point& location) {}
 
 void HeadlessWindowTreeHost::OnCursorVisibilityChangedNative(bool show) {}
 
 gfx::ICCProfile HeadlessWindowTreeHost::GetICCProfileForCurrentDisplay() {
   return gfx::ICCProfile();
 }
 
 }  // namespace headless