Disable the Context Menu in the tab-modal signin dialog.

chrome/browser/ui/signin_view_controller_delegate.h

 // Copyright 2016 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_UI_SIGNIN_VIEW_CONTROLLER_DELEGATE_H_
 #define CHROME_BROWSER_UI_SIGNIN_VIEW_CONTROLLER_DELEGATE_H_
 
 #include "chrome/browser/ui/profile_chooser_constants.h"
 #include "content/public/browser/web_contents_delegate.h"
 
@@ skipping 27 matching lines @@
 
   // Either navigates back in the signin flow if the history state allows it or
   // closes the flow otherwise.
   void PerformNavigation();
 
   // This will be called by the base class to request a resize of the native
   // view hosting the content to |height|. |height| is the total height of the
   // content, in pixels.
   virtual void ResizeNativeView(int height) = 0;
 
+  // content::WebContentsDelegate:
+  bool HandleContextMenu(const content::ContextMenuParams& params) override;
+
  protected:
   SigninViewControllerDelegate(SigninViewController* signin_view_controller,
                                content::WebContents* web_contents);
   ~SigninViewControllerDelegate() override;
 
   // Notifies the SigninViewController that this instance is being deleted.
   void ResetSigninViewControllerDelegate();
 
   // content::WebContentsDelegate
   void LoadingStateChanged(content::WebContents* source,
                            bool to_different_document) override;
 
   // This will be called by this base class when the tab-modal window must be
   // closed. This should close the platform-specific window that is currently
   // showing the sign in flow or the sync confirmation dialog.
   virtual void PerformClose() = 0;
 
  private:
   bool CanGoBack(content::WebContents* web_ui_web_contents) const;
 
   SigninViewController* signin_view_controller_;  // Not owned.
   content::WebContents* web_contents_;  // Not owned.
   DISALLOW_COPY_AND_ASSIGN(SigninViewControllerDelegate);
 };
 
 #endif  // CHROME_BROWSER_UI_SIGNIN_VIEW_CONTROLLER_DELEGATE_H_