Adds button to VRShell menu mode, which lets the user manually exit WebVR presentation.

chrome/browser/android/vr_shell/vr_shell_delegate.h

Index: chrome/browser/android/vr_shell/vr_shell_delegate.h
diff --git a/chrome/browser/android/vr_shell/vr_shell_delegate.h b/chrome/browser/android/vr_shell/vr_shell_delegate.h
index a24f3087deb87e4ea7b917ef4a6f580167675834..c8a01de0a30594240c39b0f6617862f7858d97bf 100644
--- a/chrome/browser/android/vr_shell/vr_shell_delegate.h
+++ b/chrome/browser/android/vr_shell/vr_shell_delegate.h
@@ -51,13 +51,15 @@ class VrShellDelegate : public device::GvrDelegateProvider {
 
   device::GvrDeviceProvider* device_provider() { return device_provider_; }
 
+  // device::GvrDelegateProvider implementation.
+  void ExitWebVRPresent() override;
+
  private:
-  // device::GvrDelegateProvider implementation
+  // device::GvrDelegateProvider implementation.
   void SetDeviceProvider(device::GvrDeviceProvider* device_provider) override;
   void ClearDeviceProvider() override;
   void RequestWebVRPresent(device::mojom::VRSubmitFrameClientPtr submit_client,
                            const base::Callback<void(bool)>& callback) override;
-  void ExitWebVRPresent() override;
   device::GvrDelegate* GetDelegate() override;
   void SetListeningForActivate(bool listening) override;