Disable back button when stack is empty

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

Index: chrome/browser/android/vr_shell/ui_elements/url_bar.h
diff --git a/chrome/browser/android/vr_shell/ui_elements/url_bar.h b/chrome/browser/android/vr_shell/ui_elements/url_bar.h
index 07552fc3bff9f83afb87e914c5e5a498de4c7936..15bf9943ccedbf310294d0627f055421f56ad663 100644
--- a/chrome/browser/android/vr_shell/ui_elements/url_bar.h
+++ b/chrome/browser/android/vr_shell/ui_elements/url_bar.h
@@ -35,6 +35,7 @@ class UrlBar : public TexturedElement {
   void OnBeginFrame(const base::TimeTicks& begin_frame_time) override;
   void SetEnabled(bool enabled) override;
 
+  void SetHistoryButtonsEnabled(bool can_go_back, bool can_go_forward);
   void SetURL(const GURL& gurl);
   void SetSecurityLevel(security_state::SecurityLevel level);
   void SetBackButtonCallback(const base::Callback<void()>& callback);