Add WindowTreeHostPlatform::GetHost for Android build

ui/aura/window_tree_host_platform.cc

 // Copyright 2015 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_platform.h"
 
 #include "base/trace_event/trace_event.h"
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/compositor/compositor.h"
 #include "ui/events/event.h"
 #include "ui/gfx/screen.h"
 
 #if defined(OS_ANDROID)
 #include "ui/platform_window/android/platform_window_android.h"
 #endif
 
 #if defined(USE_OZONE)
 #include "ui/ozone/public/ozone_platform.h"
 #endif
 
 #if defined(OS_WIN)
 #include "base/message_loop/message_loop.h"
 #include "ui/base/cursor/cursor_loader_win.h"
 #include "ui/platform_window/win/win_window.h"
 #endif
 
 namespace aura {
+#if defined(OS_ANDROID)
+namespace {
+  WindowTreeHostPlatform* instance = nullptr;
+}
+
+// static
+WindowTreeHostPlatform* WindowTreeHostPlatform::GetHost() {
+  return instance;
+}
+#endif
 
 // static
 WindowTreeHost* WindowTreeHost::Create(const gfx::Rect& bounds) {
+#if defined(OS_ANDROID)
+  DCHECK(!instance);
+  instance = new WindowTreeHostPlatform(bounds);
+  return instance;
+#else
   return new WindowTreeHostPlatform(bounds);
+#endif
 }
 
 WindowTreeHostPlatform::WindowTreeHostPlatform(const gfx::Rect& bounds)
     : widget_(gfx::kNullAcceleratedWidget),
       current_cursor_(ui::kCursorNull),
       has_capture_(false) {
 #if defined(USE_OZONE)
   window_ =
       ui::OzonePlatform::GetInstance()->CreatePlatformWindow(this, bounds);
 #elif defined(OS_WIN)
@@ skipping 131 matching lines @@
   DCHECK_EQ(widget, widget_);
   widget_ = gfx::kNullAcceleratedWidget;
 }
 
 void WindowTreeHostPlatform::OnActivationChanged(bool active) {
   if (active)
     OnHostActivated();
 }
 
 }  // namespace aura