Fix handling of external protocols with PlzNavigate.

url/url_util.h

Index: url/url_util.h
diff --git a/url/url_util.h b/url/url_util.h
index d0a8f22b6ff9621479d6208d0c92abf6689695f0..643c29d83fbaaab3ca7e3ddeeab0a505b2c52435 100644
--- a/url/url_util.h
+++ b/url/url_util.h
@@ -109,6 +109,11 @@ URL_EXPORT const std::vector<std::string>& GetWebStorageSchemes();
 URL_EXPORT void AddCSPBypassingScheme(const char* new_scheme);
 URL_EXPORT const std::vector<std::string>& GetCSPBypassingSchemes();
 
+// Adds an application-defined scheme to the list of schemes that are strictly
+// empty documents, allowing them to commit synchronously.
+URL_EXPORT void AddEmptyDocumentScheme(const char* new_scheme);
+URL_EXPORT const std::vector<std::string>& GetEmptyDocumentSchemes();
+
 // Sets a flag to prevent future calls to Add*Scheme from succeeding.
 //
 // This is designed to help prevent errors for multithreaded applications.