Extend Fallback from prerender requests to preconnect for non-omnibox requests

Fallback prerender requests to preconnect when it's not possible
to do a prerender. This primarily affects low memory devices.
This is behind kPrerenderFallbackToPreconnect feature.
Currently, this fallback is enabled for only the prerenders initiated by
omnibox. This CL extends it to other cases as well.

Change-Id: Iaf51b73d10a6c8f90f06ab4629ecf7d13f4a992f
Bug: 943442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1920430Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716355}

chrome/browser/prerender/prerender_manager.cc

@@ -787,11 +787,6 @@ void PrerenderManager::MaybePreconnect(Origin origin,
     return;
   }
 
-  // Currently, the fallback is only enabled for prerenders initiated by
-  // omnibox.
-  if (origin != ORIGIN_OMNIBOX)
-    return;
-
   auto* loading_predictor = predictors::LoadingPredictorFactory::GetForProfile(
       Profile::FromBrowserContext(profile_));
   if (loading_predictor) {

chrome/browser/prerender/prerender_unittest.cc

@@ -1246,7 +1246,7 @@ TEST_F(PrerenderTest, OmniboxAllowedWhenNotDisabled_LowMemory_FeatureDisabled) {
 // Test that when prerender fails and the
 // kPrerenderFallbackToPreconnect experiment is enabled, a
 // prerender initiated by omnibox actually results in preconnect.
-TEST_F(PrerenderTest, OmniboxAllowedWhenNotDisabled_LowMemory_FeatureEnabled) {
+TEST_F(PrerenderTest, Omnibox_AllowedWhenNotDisabled_LowMemory_FeatureEnabled) {
   const GURL kURL(GURL("http://www.example.com"));
   base::test::ScopedFeatureList scoped_feature_list;
   scoped_feature_list.InitAndEnableFeature(
@@ -1273,6 +1273,37 @@ TEST_F(PrerenderTest, OmniboxAllowedWhenNotDisabled_LowMemory_FeatureEnabled) {
   EXPECT_NE(it, active_hints.end());
 }
 
+// Test that when prerender fails and the
+// kPrerenderFallbackToPreconnect experiment is enabled, a
+// prerender initiated by an external request actually results in preconnect.
+TEST_F(PrerenderTest,
+       ExternalRequest_AllowedWhenNotDisabled_LowMemory_FeatureEnabled) {
+  const GURL kURL(GURL("http://www.example.com"));
+  base::test::ScopedFeatureList scoped_feature_list;
+  scoped_feature_list.InitAndEnableFeature(
+      features::kPrerenderFallbackToPreconnect);
+
+  predictors::LoadingPredictorConfig config;
+  PopulateTestConfig(&config);
+
+  auto* loading_predictor = predictors::LoadingPredictorFactory::GetForProfile(
+      Profile::FromBrowserContext(profile()));
+  loading_predictor->StartInitialization();
+  content::RunAllTasksUntilIdle();
+
+  // Prerender should be disabled on low memory devices.
+  prerender_manager()->SetIsLowEndDevice(true);
+  EXPECT_FALSE(prerender_manager()->AddPrerenderFromExternalRequest(
+      kURL, content::Referrer(), nullptr, gfx::Rect(kSize)));
+
+  // Verify that the prerender request falls back to a preconnect request.
+  EXPECT_EQ(1u, loading_predictor->GetActiveHintsSizeForTesting());
+
+  auto& active_hints = loading_predictor->active_hints_for_testing();
+  auto it = active_hints.find(kURL);
+  EXPECT_NE(it, active_hints.end());
+}
+
 TEST_F(PrerenderTest, LinkRelStillAllowedWhenDisabled) {
   DisablePrerender();
   GURL url("http://www.google.com/");