Ensure IsolatedPrerender Prefetches Don't Fallback to Direct

Added test.

Bug: 1042829
Change-Id: I51be6e4286a317dd612e91400895988302fe2bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125951
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754922}

chrome/browser/prerender/isolated/isolated_prerender_browsertest.cc

@@ -418,6 +418,13 @@ IN_PROC_BROWSER_TEST_F(IsolatedPrerenderBrowserTest,
   // This run loop will quit when a valid CONNECT request is made to the proxy
   // server.
   run_loop.Run();
+
+  // The embedded test server will return a 400 for all CONNECT requests by
+  // default. Ensure that the request didn't fallback to a direct connection.
+  IsolatedPrerenderTabHelper* tab_helper =
+      IsolatedPrerenderTabHelper::FromWebContents(GetWebContents());
+  EXPECT_EQ(tab_helper->metrics().prefetch_attempted_count, 1U);
+  EXPECT_EQ(tab_helper->metrics().prefetch_successful_count, 0U);
 }
 
 class ProbingEnabledIsolatedPrerenderBrowserTest

chrome/browser/prerender/isolated/isolated_prerender_tab_helper.cc

@@ -281,6 +281,9 @@ void IsolatedPrerenderTabHelper::OnPrefetchComplete(
       page_->url_loader_->ResponseInfo()) {
     network::mojom::URLResponseHeadPtr head =
         page_->url_loader_->ResponseInfo()->Clone();
+
+    DCHECK(!head->proxy_server.is_direct());
+
     HandlePrefetchResponse(url, key, std::move(head), std::move(body));
   }
   Prefetch();