NoStatePrefetch should prefetch WebFonts

Currently, NoStatePrefetch only fetches HTML + CSS + Sync JS.We should
experiment with also prefetching WebFonts since WebFonts are render
blocking on fast connections. This CL adds a finch parameter to LWNSP
experiment.

Bug: 1021749
Change-Id: I9bbf3c629d620df89aa6ae54d3352d7ba1fb6952
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901590Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714918}

chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc

@@ -482,6 +482,43 @@ IN_PROC_BROWSER_TEST_F(HTMLCSSSyncScriptNoStatePrefetchBrowserTest,
   WaitForRequestCount(src_server()->GetURL(kPrefetchFont), 0);
 }
 
+// Test and Test Class for lightweight prefetch under the
+// HTML+CSS+SyncScript+Font configuration.
+class HTMLCSSSyncScriptFontNoStatePrefetchBrowserTest
+    : public NoStatePrefetchBrowserTest {
+ public:
+  void SetUp() override {
+    std::map<std::string, std::string> parameters;
+    parameters["skip_other"] = "true";
+    parameters["skip_async_script"] = "true";
+    parameters["skip_font"] = "false";
+    feature_list_.InitWithFeaturesAndParameters(
+        {{blink::features::kLightweightNoStatePrefetch, parameters}}, {});
+    NoStatePrefetchBrowserTest::SetUp();
+  }
+
+ private:
+  base::test::ScopedFeatureList feature_list_;
+};
+
+// Checks that the expected resource types are fetched via NoState Prefetch.
+IN_PROC_BROWSER_TEST_F(HTMLCSSSyncScriptFontNoStatePrefetchBrowserTest,
+                       PrefetchHTMLCSSSyncScript) {
+  std::unique_ptr<TestPrerender> test_prerender =
+      PrefetchFromFile(kPrefetchPageMultipleResourceTypes,
+                       FINAL_STATUS_NOSTATE_PREFETCH_FINISHED);
+
+  // Verify that the page load did not happen.
+  test_prerender->WaitForLoads(0);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchPageMultipleResourceTypes),
+                      1);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchScript), 1);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchScript2), 0);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchPng), 0);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchCss), 1);
+  WaitForRequestCount(src_server()->GetURL(kPrefetchFont), 1);
+}
+
 // Test and Test Class for lightweight prefetch under the HTML+CSS+Script
 // configuration.
 class HTMLCSSScriptNoStatePrefetchBrowserTest

third_party/blink/renderer/core/html/parser/html_resource_preloader.cc

@@ -106,7 +106,6 @@ bool HTMLResourcePreloader::AllowPreloadRequest(PreloadRequest* preload) const {
   // resources are either classified into CSS (always fetched when not in the
   // HTML only arm), JS (skip_script param), or other.
   switch (preload->GetResourceType()) {
-    case ResourceType::kFont:
     case ResourceType::kRaw:
     case ResourceType::kSVGDocument:
     case ResourceType::kXSLStyleSheet:
@@ -123,6 +122,9 @@ bool HTMLResourcePreloader::AllowPreloadRequest(PreloadRequest* preload) const {
       return false;
     case ResourceType::kCSSStyleSheet:
       return true;
+    case ResourceType::kFont:
+      return !GetFieldTrialParamByFeatureAsBool(
+          features::kLightweightNoStatePrefetch, "skip_font", true);
     case ResourceType::kScript:
       // We might skip all script.
       if (GetFieldTrialParamByFeatureAsBool(