Always default initialize WebURLError::extended_reason_ to 0.

I think it's better to do this directly on the field decl
in the header than with every single constructor since
it's easy to forget it there: at the moment it's missing
from the third constructor in the .cc. It's possible that this
makes no difference thus far but it's better to initialize it
to avoid undefined behavior.

R=caseq

Change-Id: Ifd3e6b320bae593f9400f30ca7dc4769c391afb5
Reviewed-on: https://chromium-review.googlesource.com/1038820Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555662}

third_party/blink/public/platform/web_url_error.h

@@ -78,7 +78,7 @@ struct WebURLError {
   int reason_;
 
   // Additional information based on the reason_.
-  int extended_reason_;
+  int extended_reason_ = 0;
 
   // A flag showing whether or not we have a (possibly stale) copy of the
   // requested resource in the cache.

third_party/blink/renderer/platform/exported/web_url_error.cc

@@ -9,7 +9,7 @@
 namespace blink {
 
 WebURLError::WebURLError(int reason, const WebURL& url)
-    : reason_(reason), extended_reason_(0), url_(url) {
+    : reason_(reason), url_(url) {
   DCHECK_NE(reason_, 0);
 }