Enable (optional) blocking of webrequests in case a PAC script cannot be fetched or is invalid.

chrome/browser/extensions/extension_proxy_api_constants.h

Index: chrome/browser/extensions/extension_proxy_api_constants.h
diff --git a/chrome/browser/extensions/extension_proxy_api_constants.h b/chrome/browser/extensions/extension_proxy_api_constants.h
index 13fbe675013bc08df23789ccfc42cbbcf509bca7..1b8a99d9e7f7c2493e24136210eef4fd2cd9622e 100644
--- a/chrome/browser/extensions/extension_proxy_api_constants.h
+++ b/chrome/browser/extensions/extension_proxy_api_constants.h
@@ -13,6 +13,7 @@ namespace extension_proxy_api_constants {
 // String literals in dictionaries used to communicate with extension.
 extern const char kProxyConfigMode[];
 extern const char kProxyConfigPacScript[];
+extern const char kProxyConfigPacScriptMandatory[];
 extern const char kProxyConfigPacScriptUrl[];
 extern const char kProxyConfigPacScriptData[];
 extern const char kProxyConfigRules[];