Exit VR when our main content tab is hidden (Usually due to links opening in new tab)

chrome/browser/android/vr_shell/vr_web_contents_observer.h

 // 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.
 
 #ifndef CHROME_BROWSER_ANDROID_VR_SHELL_VR_WEB_CONTENTS_OBSERVER_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_VR_WEB_CONTENTS_OBSERVER_H_
 
 #include "base/macros.h"
 #include "content/public/browser/web_contents_observer.h"
 
@@ skipping 22 matching lines @@
   void DidStopLoading() override;
   void DidStartNavigation(
       content::NavigationHandle* navigation_handle) override;
   void DidRedirectNavigation(
       content::NavigationHandle* navigation_handle) override;
   void DidFinishNavigation(
       content::NavigationHandle* navigation_handle) override;
   void DidToggleFullscreenModeForTab(
       bool entered_fullscreen, bool will_cause_resize) override;
   void WebContentsDestroyed() override;
+  void WasHidden() override;
 
   // This class does not own the UI interface.
   UiInterface* ui_interface_;
   VrShell* vr_shell_;
 
   DISALLOW_COPY_AND_ASSIGN(VrWebContentsObserver);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_VR_WEB_CONTENTS_OBSERVER_H_