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 <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#606988}
diff --git a/base/task/task_features.cc b/base/task/task_features.cc
index 8086807..f48894b9 100644
--- a/base/task/task_features.cc
+++ b/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
\ No newline at end of file
+}  // namespace base