Make Page Visibility API work when the browser window is visible or not

ui/aura/window_tree_host.cc

 // Copyright (c) 2013 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 "ui/aura/window_tree_host.h"
 
 #include "base/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/cursor_client.h"
@@ skipping 241 matching lines @@
 void WindowTreeHost::OnHostActivated() {
   Env::GetInstance()->NotifyHostActivated(this);
 }
 
 void WindowTreeHost::OnHostLostWindowCapture() {
   Window* capture_window = client::GetCaptureWindow(window());
   if (capture_window && capture_window->GetRootWindow() == window())
     capture_window->ReleaseCapture();
 }
 
+void WindowTreeHost::OnHostSetWindowVisibility(bool visible) {
+  FOR_EACH_OBSERVER(WindowTreeHostObserver, observers_,
+      OnHostSetWindowVisibility(this, visible));
+}
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHost, private:
 
 void WindowTreeHost::MoveCursorToInternal(const gfx::Point& root_location,
                                           const gfx::Point& host_location) {
   last_cursor_request_position_in_host_ = host_location;
   MoveCursorToNative(host_location);
   client::CursorClient* cursor_client = client::GetCursorClient(window());
   if (cursor_client) {
     const gfx::Display& display =
         gfx::Screen::GetScreenFor(window())->GetDisplayNearestWindow(window());
     cursor_client->SetDisplay(display);
   }
   dispatcher()->OnCursorMovedToRootLocation(root_location);
 }
 
 }  // namespace aura