Initialize ResourceError::extended_error_code_ to zero

Not all constructors write this field, so defaulting to zero
may some day help avoid grief from uninitialized values or
undefined behavior.

Change-Id: I478ecbc346a78d68721e3731f9cec9aad77919fa
Reviewed-on: https://chromium-review.googlesource.com/1054611
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557963}

third_party/blink/renderer/platform/loader/fetch/resource_error.h

@@ -97,7 +97,7 @@ class PLATFORM_EXPORT ResourceError final {
   void InitializeDescription();
 
   int error_code_;
-  int extended_error_code_;
+  int extended_error_code_ = 0;
   KURL failing_url_;
   String localized_description_;
   bool is_access_check_ = false;