LoadingPredictor: Set referrer policy of prefetch requests to no-referrer

The prefetch can happen before the referrer policy is known, so this CL
makes the predictor use a conservertive one (no-referrer) by default.

Bug: 1107318
Change-Id: If8c27090d5530edd457c83c23a059a76b90ad446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2331599Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793543}

chrome/browser/predictors/prefetch_manager.cc

@@ -206,7 +206,10 @@ void PrefetchManager::PrefetchUrl(
   request.url = job->url;
   request.site_for_cookies = net::SiteForCookies::FromUrl(info.url);
   request.request_initiator = top_frame_origin;
-  request.referrer = info.url;
+
+  // The prefetch can happen before the referrer policy is known, so use a
+  // conservative one (no-referrer) by default.
+  request.referrer_policy = net::ReferrerPolicy::NO_REFERRER;
 
   request.headers.SetHeader("Purpose", "prefetch");
 

chrome/browser/predictors/prefetch_manager_unittest.cc

@@ -144,6 +144,7 @@ TEST_F(PrefetchManagerTest, OneMainFrameUrlOnePrefetch) {
         EXPECT_EQ(request.url, subresource_url);
         EXPECT_TRUE(request.load_flags & net::LOAD_PREFETCH);
 
+        EXPECT_EQ(request.referrer_policy, net::ReferrerPolicy::NO_REFERRER);
         EXPECT_EQ(request.destination,
                   network::mojom::RequestDestination::kScript);
         EXPECT_EQ(