Disable back button when stack is empty

chrome/browser/android/vr_shell/textures/url_bar_texture.h

 // Copyright 2017 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_TEXTURES_URL_BAR_TEXTURE_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_TEXTURES_URL_BAR_TEXTURE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 21 matching lines @@
     FLAG_BACK_DOWN = 1 << 1,
   };
 
   explicit UrlBarTexture(
       const base::Callback<void(UiUnsupportedMode)>& failure_callback);
   ~UrlBarTexture() override;
   gfx::Size GetPreferredTextureSize(int width) const override;
   gfx::SizeF GetDrawnSize() const override;
 
   void SetURL(const GURL& gurl);
+  void SetHistoryButtonsEnabled(bool can_go_back);
   void SetSecurityLevel(security_state::SecurityLevel level);
 
   bool HitsBackButton(const gfx::PointF& position) const;
   bool HitsUrlBar(const gfx::PointF& position) const;
 
   void SetHovered(bool hovered);
   void SetPressed(bool pressed);
 
   // Public for testability.
   static void ApplyUrlStyling(const base::string16& formatted_url,
                               const url::Parsed& parsed,
                               security_state::SecurityLevel security_level,
                               vr_shell::RenderTextWrapper* render_text,
                               const ColorScheme& color_scheme);
 
  private:
   void Draw(SkCanvas* canvas, const gfx::Size& texture_size) override;
   float ToPixels(float meters) const;
   bool HitsTransparentRegion(const gfx::PointF& meters, bool left) const;
   void RenderUrl(const gfx::Size& texture_size, const gfx::Rect& bounds);
   void OnSetMode() override;
   const ColorScheme& color_scheme() const;
 
   gfx::SizeF size_;
   bool hovered_ = false;
   bool pressed_ = false;
+  bool can_go_back_ = false;
 
   GURL gurl_;
   security_state::SecurityLevel security_level_;
 
   std::unique_ptr<gfx::RenderText> url_render_text_;
   GURL last_drawn_gurl_;
   security_state::SecurityLevel last_drawn_security_level_;
   base::Callback<void(UiUnsupportedMode)> failure_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(UrlBarTexture);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_TEXTURES_URL_BAR_TEXTURE_H_