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

net/proxy/proxy_service.cc

Index: net/proxy/proxy_service.cc
diff --git a/net/proxy/proxy_service.cc b/net/proxy/proxy_service.cc
index ca3629ebcdcdb6361921b2daf14d0b1bd7acd984..c142a903ae3fc264f77f59be6d08d982b4872e57 100644
--- a/net/proxy/proxy_service.cc
+++ b/net/proxy/proxy_service.cc
@@ -552,6 +552,9 @@ int ProxyService::TryToCompleteSynchronously(const GURL& url,
 
   DCHECK_NE(config_.id(), ProxyConfig::INVALID_ID);
 
+  if (config_.block_everything())
+    return ERR_PROXY_CONNECTION_FAILED;
+
   if (config_.HasAutomaticSettings())
     return ERR_IO_PENDING;  // Must submit the request to the proxy resolver.
 
@@ -642,10 +645,17 @@ void ProxyService::OnInitProxyResolverComplete(int result) {
   init_proxy_resolver_.reset();
 
   if (result != OK) {
-    VLOG(1) << "Failed configuring with PAC script, falling-back to manual "
-               "proxy servers.";
-    config_ = fetched_config_;
-    config_.ClearAutomaticSettings();
+    if (fetched_config_.pac_mandatory()) {
+      VLOG(1) << "Failed configuring with mandatory PAC script, blocking all "
+                 "traffic.";
+      config_ = fetched_config_;
+      config_.set_block_everything(true);
+    } else {
+      VLOG(1) << "Failed configuring with PAC script, falling-back to manual "
+                 "proxy servers.";
+      config_ = fetched_config_;
+      config_.ClearAutomaticSettings();
+    }
   }
 
   config_.set_id(fetched_config_.id());
@@ -723,15 +733,19 @@ int ProxyService::DidFinishResolvingProxy(ProxyInfo* result,
         make_scoped_refptr(new NetLogIntegerParameter(
             "net_error", result_code)));
 
-    // Fall-back to direct when the proxy resolver fails. This corresponds
-    // with a javascript runtime error in the PAC script.
-    //
-    // This implicit fall-back to direct matches Firefox 3.5 and
-    // Internet Explorer 8. For more information, see:
-    //
-    // http://www.chromium.org/developers/design-documents/proxy-settings-fallback
-    result->UseDirect();
-    result_code = OK;
+    if (!config_.pac_mandatory()) {
+      // Fall-back to direct when the proxy resolver fails. This corresponds
+      // with a javascript runtime error in the PAC script.
+      //
+      // This implicit fall-back to direct matches Firefox 3.5 and
+      // Internet Explorer 8. For more information, see:
+      //
+      // http://www.chromium.org/developers/design-documents/proxy-settings-fallback
+      result->UseDirect();
+      result_code = OK;
+    } else {
+      result_code = ERR_PROXY_CONNECTION_FAILED;
+    }
   }
 
   net_log.EndEvent(NetLog::TYPE_PROXY_SERVICE, NULL);