loader: Make Resource::LastResourceResponse() work when no redirects.

Change-Id: I30dd3f55cc8832512f52062d9b5bff085887f392
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415918Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809636}

third_party/blink/renderer/core/loader/resource_load_observer_for_frame.cc

@@ -155,12 +155,12 @@ void ResourceLoadObserverForFrame::DidReceiveResponse(
 
     if (RuntimeEnabledFeatures::SignedExchangeSubresourcePrefetchEnabled(
             document_->GetExecutionContext()) &&
-        resource->LastResourceResponse()) {
+        resource->RedirectChainSize() > 0) {
       // See if the outer response (which must be the last response in
       // the redirect chain) had provided alternate links for the prefetch.
       alternate_resource_info =
           AlternateSignedExchangeResourceInfo::CreateIfValid(
-              resource->LastResourceResponse()->HttpHeaderField(
+              resource->LastResourceResponse().HttpHeaderField(
                   http_names::kLink),
               response.HttpHeaderField(http_names::kLink));
     }

third_party/blink/renderer/platform/loader/fetch/resource.cc

@@ -477,10 +477,14 @@ const ResourceRequestHead& Resource::LastResourceRequest() const {
   return redirect_chain_.back().request_;
 }
 
-const ResourceResponse* Resource::LastResourceResponse() const {
+const ResourceResponse& Resource::LastResourceResponse() const {
   if (!redirect_chain_.size())
-    return nullptr;
-  return &redirect_chain_.back().redirect_response_;
+    return GetResponse();
+  return redirect_chain_.back().redirect_response_;
+}
+
+size_t Resource::RedirectChainSize() const {
+  return redirect_chain_.size();
 }
 
 void Resource::SetRevalidatingRequest(const ResourceRequestHead& request) {

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

@@ -169,7 +169,9 @@ class PLATFORM_EXPORT Resource : public GarbageCollected<Resource>,
     return resource_request_;
   }
   const ResourceRequestHead& LastResourceRequest() const;
-  const ResourceResponse* LastResourceResponse() const;
+  const ResourceResponse& LastResourceResponse() const;
+  // Returns zero if there are no redirects.
+  size_t RedirectChainSize() const;
 
   virtual void SetRevalidatingRequest(const ResourceRequestHead&);