Turn tabdiscarder off.

This will stop TabManagerDelete to call TabActivityWatcher, hence
disable tabdiscarder completely.

Note: we can still do experiment from Finch.

Bug: 1017222
Change-Id: I064004ae6e0e67d2df0133c98626c395db73f6db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888863
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710830}

chrome/browser/resource_coordinator/tab_manager_delegate_chromeos_unittest.cc

@@ -109,7 +109,9 @@ TEST_F(TabManagerDelegateTest, CandidatesSortedWithFocusedAppAndTab) {
 // is turned on.
 TEST_F(TabManagerDelegateTest, SortLifecycleUnitWithTabRanker) {
   base::test::ScopedFeatureList feature_list;
-  feature_list.InitAndEnableFeature(features::kTabRanker);
+  feature_list.InitAndEnableFeatureWithParameters(
+      features::kTabRanker,
+      {{"number_of_oldest_tabs_to_score_with_TabRanker", "20"}});
   std::vector<arc::ArcProcess> arc_processes;
   arc_processes.emplace_back(1, 10, "focused", arc::mojom::ProcessState::TOP,
                              kIsFocused, 99);

chrome/browser/resource_coordinator/tab_manager_features.cc

@@ -257,8 +257,7 @@ GetStaticSiteCharacteristicsDatabaseParams() {
 
 int GetNumOldestTabsToScoreWithTabRanker() {
   return base::GetFieldTrialParamByFeatureAsInt(
-      features::kTabRanker, "number_of_oldest_tabs_to_score_with_TabRanker",
-      20);
+      features::kTabRanker, "number_of_oldest_tabs_to_score_with_TabRanker", 0);
 }
 
 int GetProcessTypeToScoreWithTabRanker() {