Prerender: Remove unnecessary abort code from LinkLoader::LoadLink()

LinkLoader::LoadLink() aborts inflight prerendering by calling Abort()
at the beginning of the function, so it doesn't have to run another
abort sequence later.

This is tested by the existing PrerenderTest.MutateTarget test case.

Bug: n/a
Change-Id: Ibc70b94c0acaae52eb4f7d27f1a65210c15f9c10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426664Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810060}

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

@@ -199,18 +199,10 @@ bool LinkLoader::LoadLink(const LinkLoadParameters& params,
   base::Optional<mojom::blink::PrerenderRelType> prerender_rel_type =
       PrerenderRelTypeFromRelAttribute(params.rel, document);
   if (prerender_rel_type) {
-    if (!prerender_) {
-      prerender_ = PrerenderHandle::Create(document, this, params.href,
-                                           *prerender_rel_type);
-    } else if (prerender_->Url() != params.href) {
-      prerender_->Cancel();
-      prerender_ = PrerenderHandle::Create(document, this, params.href,
-                                           *prerender_rel_type);
-    }
-    // TODO(gavinp): Handle changes to rel types of existing prerenders.
-  } else if (prerender_) {
-    prerender_->Cancel();
-    prerender_.Clear();
+    // The previous prerender should already be aborted by Abort().
+    DCHECK(!prerender_);
+    prerender_ = PrerenderHandle::Create(document, this, params.href,
+                                         *prerender_rel_type);
   }
   return true;
 }