Launching lightweight no state prefetch on Android

We experimented with this feature on Android only, and determined it is
worth launching. We will explore this space more in the future
(especially Desktop experiments).

The launching arm is slightly different than what was in
fieldtrial_testing_config.json as HTML+CSS+SyncScript performed best
in Stable.

Bug: 934466
Change-Id: Id5d4e5ea9558133704639a5cc902e520a2bf6370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838756Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Auto-Submit: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702945}

chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc

@@ -490,6 +490,7 @@ class HTMLCSSScriptNoStatePrefetchBrowserTest
   void SetUp() override {
     std::map<std::string, std::string> parameters;
     parameters["skip_other"] = "true";
+    parameters["skip_async_script"] = "false";
     feature_list_.InitWithFeaturesAndParameters(
         {{blink::features::kLightweightNoStatePrefetch, parameters}}, {});
     NoStatePrefetchBrowserTest::SetUp();

testing/variations/fieldtrial_testing_config.json

@@ -3281,25 +3281,6 @@
             ]
         }
     ],
-    "LightweightNoStatePrefetch": [
-        {
-            "platforms": [
-                "android"
-            ],
-            "experiments": [
-                {
-                    "name": "HTML_CSS_SyncScript_20190725",
-                    "params": {
-                        "skip_async_script": "true",
-                        "skip_other": "true"
-                    },
-                    "enable_features": [
-                        "LightweightNoStatePrefetch"
-                    ]
-                }
-            ]
-        }
-    ],
     "LoadingOnlyLearnHighPriorityResources": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -293,7 +293,13 @@ const base::Feature kAudioWorkletRealtimeThread{
 
 // A feature to reduce the set of resources fetched by No-State Prefetch.
 const base::Feature kLightweightNoStatePrefetch{
-    "LightweightNoStatePrefetch", base::FEATURE_DISABLED_BY_DEFAULT};
+  "LightweightNoStatePrefetch",
+#if defined(OS_ANDROID)
+      base::FEATURE_ENABLED_BY_DEFAULT
+#else
+      base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
 
 // Use scroll gestures for scrollbar scrolls (see https://crbug.com/954007).
 const base::Feature kScrollbarInjectScrollGestures{

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

@@ -118,7 +118,7 @@ bool HTMLResourcePreloader::AllowPreloadRequest(PreloadRequest* preload) const {
     case ResourceType::kManifest:
     case ResourceType::kMock:
       return !GetFieldTrialParamByFeatureAsBool(
-          features::kLightweightNoStatePrefetch, "skip_other", false);
+          features::kLightweightNoStatePrefetch, "skip_other", true);
     case ResourceType::kImage:
       return false;
     case ResourceType::kCSSStyleSheet:
@@ -133,7 +133,7 @@ bool HTMLResourcePreloader::AllowPreloadRequest(PreloadRequest* preload) const {
       // Otherwise, we might skip async/deferred script.
       return !GetFieldTrialParamByFeatureAsBool(
                  features::kLightweightNoStatePrefetch, "skip_async_script",
-                 false) ||
+                 true) ||
              preload->DeferOption() == FetchParameters::DeferOption::kNoDefer;
   }
 }