Referrer: Return null string when outgoing_referrer is an empty string

This CL changes the constructor of blink::WebFetchClientSettingsObject
to set blink::WebFetchClientSettingsObject::outgoing_referrer to a null
string when given outgoing_referrer argument is a non-null empty string.
outgoing_referrer is not allowed to take a non-null empty string (i.e. String("")).
However, null string are converted into non-null empty string when
outgoing_referrer is passed via mojo which takes url.mojom.Url.
To keep prohibiting blink::WebFetchClientSettingsObject::outgoing_referrer
to be non-null empty string, this CL converts empty strings back
to null string at constructors.

Bug: 1047612
Change-Id: Iac8ea9ca4e752454f524e6ee28a9dce932ca572c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033004
Commit-Queue: Eriko Kurimoto <elkurin@google.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738070}

third_party/blink/public/platform/web_fetch_client_settings_object.h

@@ -23,6 +23,9 @@ namespace blink {
 struct WebFetchClientSettingsObject {
   network::mojom::ReferrerPolicy referrer_policy =
       network::mojom::ReferrerPolicy::kDefault;
+  // outgoing_referrer must be either invalid (!IsValid()) or a valid, non-empty
+  // WebURL (IsValid() && !IsEmpty()).
+  // See https://crbug.com/1047612.
   WebURL outgoing_referrer;
   mojom::InsecureRequestsPolicy insecure_requests_policy =
       blink::mojom::InsecureRequestsPolicy::kDoNotUpgrade;
@@ -32,14 +35,26 @@ struct WebFetchClientSettingsObject {
       WebURL outgoing_referrer,
       mojom::InsecureRequestsPolicy insecure_requests_policy)
       : referrer_policy(referrer_policy),
-        outgoing_referrer(outgoing_referrer),
+        // As per the comment on the |outgoing_referrer| member, it cannot be
+        // set to a valid, empty WebURL. But the given |outgoing_referrer| may
+        // be a valid, empty WebURL, for example, due to conversion from
+        // mojom::Url that doesn't distinguish between null and empty. We
+        // canonicalize it to an invalid WebURL if it's empty.
+        outgoing_referrer(outgoing_referrer.IsEmpty() ? WebURL()
+                                                      : outgoing_referrer),
         insecure_requests_policy(insecure_requests_policy) {}
 
 #if INSIDE_BLINK
   explicit WebFetchClientSettingsObject(
       const FetchClientSettingsObject& settings_object)
       : referrer_policy(settings_object.GetReferrerPolicy()),
-        outgoing_referrer(KURL(settings_object.GetOutgoingReferrer())),
+        // As per the comment on the |outgoing_referrer| member, it cannot be
+        // set to a valid, empty WebURL. But the given |outgoing_referrer| may
+        // be a non-null empty String since the caller may pass one.
+        // We canonicalize it to an invalid WebURL if it's empty.
+        outgoing_referrer(settings_object.GetOutgoingReferrer().IsEmpty()
+                              ? KURL()
+                              : KURL(settings_object.GetOutgoingReferrer())),
         insecure_requests_policy(
             settings_object.GetInsecureRequestsPolicy() &
                     blink::kUpgradeInsecureRequests