Add a browser test for the views external protocol dialog.

chrome/browser/ui/views/external_protocol_dialog.h

Index: chrome/browser/ui/views/external_protocol_dialog.h
diff --git a/chrome/browser/ui/views/external_protocol_dialog.h b/chrome/browser/ui/views/external_protocol_dialog.h
index ad2efc329741f562b0a4e07e125e990a80e51dbb..3c53e7d2d12ed5a70cfcca9251d46c4ec501613f 100644
--- a/chrome/browser/ui/views/external_protocol_dialog.h
+++ b/chrome/browser/ui/views/external_protocol_dialog.h
@@ -13,6 +13,10 @@
 
 class ProtocolDialogDelegate;
 
+namespace test {
+class ExternalProtocolDialogTestApi;
+}
+
 namespace views {
 class MessageBoxView;
 }
@@ -40,6 +44,8 @@ class ExternalProtocolDialog : public views::DialogDelegate {
   ui::ModalType GetModalType() const override;
 
  private:
+  friend class test::ExternalProtocolDialogTestApi;
+
   const std::unique_ptr<const ProtocolDialogDelegate> delegate_;
 
   // The message box view whose commands we handle.