[vr] Close exit prompt when clicking on background

chrome/browser/android/vr_shell/ui_elements/ui_element_debug_id.h

 // Copyright 2017 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 CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_UI_ELEMENT_DEBUG_ID_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_UI_ELEMENT_DEBUG_ID_H_
 
 namespace vr_shell {
 
 // These identifiers may be used by UI elements to tag themselves for purposes
 // of testing or debugging.
 enum UiElementDebugId {
   kNone = 0,
   kWebVrPermanentHttpSecurityWarning,
   kWebVrTransientHttpSecurityWarning,
   kContentQuad,
   kBackplane,
   kCeiling,
   kFloor,
   kUrlBar,
   kLoadingIndicator,
   kAudioCaptureIndicator,
   kVideoCaptureIndicator,
   kScreenCaptureIndicator,
   kCloseButton,
   kScreenDimmer,
   kExitWarning,
   kExitPrompt,
+  kExitPromptBackplane,
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_UI_ELEMENT_DEBUG_ID_H_