Add ability to control StaleWhileRevalidate via OriginTrials

The implementation isn't there yet but this flag controls whether
the eventual implementation should do something or not.

BUG=348877

Change-Id: I72a5a0eb2873d0a06ae5ce84c494b4d24b91434d
Reviewed-on: https://chromium-review.googlesource.com/1100942
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567974}

third_party/blink/public/platform/web_feature.mojom

@@ -1938,6 +1938,7 @@ enum WebFeature {
   kTextToSpeech_Speak = 2471,
   kTextToSpeech_SpeakCrossOrigin = 2472,
   kTextToSpeech_SpeakDisallowedByAutoplay = 2473,
+  kStaleWhileRevalidateEnabled = 2474,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/loader/document_loader.cc

@@ -69,6 +69,7 @@
 #include "third_party/blink/renderer/core/loader/resource/script_resource.h"
 #include "third_party/blink/renderer/core/loader/subresource_filter.h"
 #include "third_party/blink/renderer/core/origin_trials/origin_trial_context.h"
+#include "third_party/blink/renderer/core/origin_trials/origin_trials.h"
 #include "third_party/blink/renderer/core/page/frame_tree.h"
 #include "third_party/blink/renderer/core/page/page.h"
 #include "third_party/blink/renderer/core/probe/core_probes.h"
@@ -1115,6 +1116,14 @@ void DocumentLoader::InstallNewDocument(
     OriginTrialContext::AddTokensFromHeader(
         document, response_.HttpHeaderField(HTTPNames::Origin_Trial));
   }
+  bool stale_while_revalidate_enabled =
+      OriginTrials::staleWhileRevalidateEnabled(document);
+  fetcher_->SetStaleWhileRevalidateEnabled(stale_while_revalidate_enabled);
+
+  // If stale while revalidate is enabled via Origin Trials count it as such.
+  if (stale_while_revalidate_enabled &&
+      !RuntimeEnabledFeatures::StaleWhileRevalidateEnabledByRuntimeFlag())
+    UseCounter::Count(frame_, WebFeature::kStaleWhileRevalidateEnabled);
 
   parser_ = document->OpenForNavigation(parsing_policy, mime_type, encoding);
 

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.cc

@@ -357,7 +357,8 @@ ResourceFetcher::ResourceFetcher(FetchContext* new_context)
       auto_load_images_(true),
       images_enabled_(true),
       allow_stale_resources_(false),
-      image_fetched_(false) {
+      image_fetched_(false),
+      stale_while_revalidate_enabled_(false) {
   InstanceCounters::IncrementCounter(InstanceCounters::kResourceFetcherCounter);
   if (IsMainThread())
     MainThreadFetchersSet().insert(this);
@@ -1716,6 +1717,10 @@ void ResourceFetcher::PrepareForLeakDetection() {
   StopFetchingIncludingKeepaliveLoaders();
 }
 
+void ResourceFetcher::SetStaleWhileRevalidateEnabled(bool enabled) {
+  stale_while_revalidate_enabled_ = enabled;
+}
+
 void ResourceFetcher::StopFetchingInternal(StopFetchingTarget target) {
   // TODO(toyoshim): May want to suspend scheduler while canceling loaders so
   // that the cancellations below do not awake unnecessary scheduling.

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.h

@@ -177,6 +177,8 @@ class PLATFORM_EXPORT ResourceFetcher
   // counting.
   void PrepareForLeakDetection();
 
+  void SetStaleWhileRevalidateEnabled(bool enabled);
+
   using ResourceFetcherSet = PersistentHeapHashSet<WeakMember<ResourceFetcher>>;
   static const ResourceFetcherSet& MainThreadFetchers();
 
@@ -312,11 +314,12 @@ class PLATFORM_EXPORT ResourceFetcher
 
   uint32_t inflight_keepalive_bytes_ = 0;
 
-  // 28 bits left
+  // 27 bits left
   bool auto_load_images_ : 1;
   bool images_enabled_ : 1;
   bool allow_stale_resources_ : 1;
   bool image_fetched_ : 1;
+  bool stale_while_revalidate_enabled_ : 1;
 
   static constexpr uint32_t kKeepaliveInflightBytesQuota = 64 * 1024;
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1190,6 +1190,11 @@
       name: "StackedCSSPropertyAnimations",
       status: "experimental",
     },
+    {
+      name: "StaleWhileRevalidate",
+      origin_trial_feature_name: "StaleWhileRevalidate",
+      status: "test",
+    },
     {
       name: "StopInBackground",
       status: "test",

tools/metrics/histograms/enums.xml

@@ -19005,6 +19005,7 @@ Called by update_net_error_codes.py.-->
   <int value="2471" label="TextToSpeech_Speak"/>
   <int value="2472" label="TextToSpeech_SpeakCrossOrigin"/>
   <int value="2473" label="TextToSpeech_SpeakDisallowedByAutoplay"/>
+  <int value="2474" label="StaleWhileRevalidateEnabled"/>
 </enum>
 
 <enum name="FeedbackSource">