TaskScheduler: Use a Feature to control whether all tasks are USER_BLOCKING.

Previously, we read a variation param from the "BrowserScheduler" study
to determine whether all tasks should have USER_BLOCKING priority.
With this CL, we instead use a base::Feature.

Benefits:
- Study that controls the feature doesn't have to be "BrowserScheduler".
- Enabling/disabling a feature is less verbose than setting a variation
  params with the new GCL config format.

Bug: 902441
Change-Id: Ib38dcfdf097c1ca92784a22c3501b644b077f744
Reviewed-on: https://chromium-review.googlesource.com/c/1321032
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606988}

base/task/task_features.cc

@@ -8,7 +8,10 @@
 
 namespace base {
 
-const base::Feature kMergeBlockingNonBlockingPools = {
+const Feature kAllTasksUserBlocking{"AllTasksUserBlocking",
+                                    FEATURE_DISABLED_BY_DEFAULT};
+
+const Feature kMergeBlockingNonBlockingPools = {
     "MergeBlockingNonBlockingPools", base::FEATURE_DISABLED_BY_DEFAULT};
 
 }  // namespace base

base/task/task_features.h

@@ -10,7 +10,9 @@
 namespace base {
 
 struct Feature;
-extern const BASE_EXPORT base::Feature kMergeBlockingNonBlockingPools;
+
+extern const BASE_EXPORT Feature kAllTasksUserBlocking;
+extern const BASE_EXPORT Feature kMergeBlockingNonBlockingPools;
 
 }  // namespace base
 

base/task/task_scheduler/task_scheduler_impl.cc

@@ -115,10 +115,8 @@ void TaskSchedulerImpl::Start(
     SchedulerWorkerObserver* scheduler_worker_observer) {
   // This is set in Start() and not in the constructor because variation params
   // are usually not ready when TaskSchedulerImpl is instantiated in a process.
-  if (base::GetFieldTrialParamValue("BrowserScheduler",
-                                    "AllTasksUserBlocking") == "true") {
+  if (FeatureList::IsEnabled(kAllTasksUserBlocking))
     all_tasks_user_blocking_.Set();
-  }
 
   const bool use_blocking_pools =
       !base::FeatureList::IsEnabled(kMergeBlockingNonBlockingPools);

base/task/task_scheduler/task_scheduler_impl_unittest.cc

@@ -248,20 +248,17 @@ std::vector<TaskSchedulerImplTestParams> GetTaskSchedulerImplTestParams() {
 class TaskSchedulerImplTest
     : public testing::TestWithParam<TaskSchedulerImplTestParams> {
  protected:
-  TaskSchedulerImplTest() : scheduler_("Test"), field_trial_list_(nullptr) {
-    if (GetParam().pool_config == PoolConfiguration::kMergeBlockingNonBlocking)
-      feature_list.InitWithFeatures({kMergeBlockingNonBlockingPools}, {});
+  TaskSchedulerImplTest() : scheduler_("Test") {
+    feature_list_.emplace();
+    feature_list_->InitWithFeatures(GetFeaturesEnabledByConstructor(), {});
   }
 
   void EnableAllTasksUserBlocking() {
-    constexpr char kFieldTrialName[] = "BrowserScheduler";
-    constexpr char kFieldTrialTestGroup[] = "DummyGroup";
-    std::map<std::string, std::string> variation_params;
-    variation_params["AllTasksUserBlocking"] = "true";
-    base::AssociateFieldTrialParams(kFieldTrialName, kFieldTrialTestGroup,
-                                    variation_params);
-    base::FieldTrialList::CreateFieldTrial(kFieldTrialName,
-                                           kFieldTrialTestGroup);
+    feature_list_.reset();
+    feature_list_.emplace();
+    std::vector<Feature> enabled_features = GetFeaturesEnabledByConstructor();
+    enabled_features.push_back(kAllTasksUserBlocking);
+    feature_list_->InitWithFeatures(enabled_features, {});
   }
 
   void set_scheduler_worker_observer(
@@ -296,8 +293,14 @@ class TaskSchedulerImplTest
   TaskSchedulerImpl scheduler_;
 
  private:
-  base::FieldTrialList field_trial_list_;
-  base::test::ScopedFeatureList feature_list;
+  std::vector<Feature> GetFeaturesEnabledByConstructor() {
+    if (GetParam().pool_config == PoolConfiguration::kMergeBlockingNonBlocking)
+      return {kMergeBlockingNonBlockingPools};
+    else
+      return {};
+  }
+
+  Optional<base::test::ScopedFeatureList> feature_list_;
   SchedulerWorkerObserver* scheduler_worker_observer_ = nullptr;
   bool did_tear_down_ = false;