Enable PageAlmostIdle by default

Also change the default value of the "mainThreadTaskLoadLowThreshold"
param to 25 as it's the value we've used in all our Finch experiments.

Bug: 829933
Change-Id: If26ad7c87e97b325ba254ce5346e6becfb353899
Reviewed-on: https://chromium-review.googlesource.com/c/1277270Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599260}

services/resource_coordinator/public/cpp/resource_coordinator_features.cc

@@ -19,7 +19,7 @@ constexpr char kMainThreadTaskLoadLowThresholdParameterName[] =
 namespace features {
 
 const base::Feature kPageAlmostIdle{"PageAlmostIdle",
-                                    base::FEATURE_DISABLED_BY_DEFAULT};
+                                    base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables CPU/memory performance measurements on PageAlmostIdle events.
 const base::Feature kPerformanceMeasurement{"PerformanceMeasurement",
@@ -34,7 +34,7 @@ bool IsPageAlmostIdleSignalEnabled() {
 }
 
 int GetMainThreadTaskLoadLowThreshold() {
-  static const int kDefaultThreshold = 30;
+  static const int kDefaultThreshold = 25;
 
   std::string value_str = base::GetFieldTrialParamValueByFeature(
       features::kPageAlmostIdle, kMainThreadTaskLoadLowThresholdParameterName);

testing/variations/fieldtrial_testing_config.json

@@ -4525,7 +4525,6 @@
                         "mainThreadTaskLoadLowThreshold": "25"
                     },
                     "enable_features": [
-                        "PageAlmostIdle",
                         "PageLifecycle",
                         "ProactiveTabFreezeAndDiscard",
                         "SiteCharacteristicsDatabase",