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

chrome/browser/ui/signin_view_controller_delegate.h

Index: chrome/browser/ui/signin_view_controller_delegate.h
diff --git a/chrome/browser/ui/signin_view_controller_delegate.h b/chrome/browser/ui/signin_view_controller_delegate.h
index a504d1340df8d0d2c0aa2280fdc87418d961a252..f2e767a4382002f71b5ea41659534ed856da3343 100644
--- a/chrome/browser/ui/signin_view_controller_delegate.h
+++ b/chrome/browser/ui/signin_view_controller_delegate.h
@@ -45,6 +45,9 @@ class SigninViewControllerDelegate : public content::WebContentsDelegate {
   // 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);