[Mac] Add cocoa implementation for sidebar extension action support

chrome/browser/ui/cocoa/browser_window_cocoa.h

Index: chrome/browser/ui/cocoa/browser_window_cocoa.h
diff --git a/chrome/browser/ui/cocoa/browser_window_cocoa.h b/chrome/browser/ui/cocoa/browser_window_cocoa.h
index 0b9295b242f74342edf121f116ede83b3305fec4..90226f839d93261843013a0a73769b13b6849fd8 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.h
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.h
@@ -179,11 +179,16 @@ class BrowserWindowCocoa
   // Returns the cocoa-world BrowserWindowController
   BrowserWindowController* cocoa_controller() { return controller_; }
 
+  // Handle Sidebar
+  void ShowSidebar(content::WebContents* sidebar_contents);
+  void HideSidebar();
+
  protected:
   void DestroyBrowser() override;
 
  private:
   NSWindow* window() const;  // Accessor for the (current) |NSWindow|.
+  void UpdateSidebarForContents(content::WebContents* sidebar_contents);
 
   Browser* browser_;  // weak, owned by controller
   BrowserWindowController* controller_;  // weak, owns us