VrShell: Exit fullscreen on app button press.

chrome/browser/android/vr_shell/vr_shell.h

Index: chrome/browser/android/vr_shell/vr_shell.h
diff --git a/chrome/browser/android/vr_shell/vr_shell.h b/chrome/browser/android/vr_shell/vr_shell.h
index 3838edfb93c4bcb584138aeadbeb155d31bdb413..135898f81ee68c2ba852b9f6171eb9d89e9bda83 100644
--- a/chrome/browser/android/vr_shell/vr_shell.h
+++ b/chrome/browser/android/vr_shell/vr_shell.h
@@ -147,6 +147,7 @@ class VrShell : public device::PresentingGvrDelegate,
   void ContentFrameWasResized(bool width_changed);
 
   void ForceExitVr();
+  void ExitFullscreen();
 
   void ProcessContentGesture(std::unique_ptr<blink::WebInputEvent> event);
   void SubmitControllerModel(std::unique_ptr<VrControllerModel> model);