VrShell: Exit fullscreen on app button press.

chrome/browser/android/vr_shell/vr_gl_thread.h

Index: chrome/browser/android/vr_shell/vr_gl_thread.h
diff --git a/chrome/browser/android/vr_shell/vr_gl_thread.h b/chrome/browser/android/vr_shell/vr_gl_thread.h
index 87bae4c514808c8ca547849dbe9f913f79ec635a..6d25bf2484a25e58112b10c4e52b5653006aa93f 100644
--- a/chrome/browser/android/vr_shell/vr_gl_thread.h
+++ b/chrome/browser/android/vr_shell/vr_gl_thread.h
@@ -50,6 +50,7 @@ class VrGLThread : public base::Thread,
   void ProcessContentGesture(
       std::unique_ptr<blink::WebInputEvent> event) override;
   void ForceExitVr() override;
+  void ExitFullscreen() override;
   void RunVRDisplayInfoCallback(
       const base::Callback<void(device::mojom::VRDisplayInfoPtr)>& callback,
       device::mojom::VRDisplayInfoPtr* info) override;