[Launcher pop-in] Add skeleton class of BurnInRanker.

This CL adds a skeleton class for a new BurnInRanker. Follow ups will
add method implementations.

Once implemented, this will be a lightweight ranker at the end of the
ranking stack. It will be responsible for two main tasks:

(1) Enforcing a "burn-in" period of some fixed number of milliseconds,
during which results are collected from providers but are not published
to the front-end. The purpose of this is to prevent the rapid and
jittery updating of the result list at the beginning of display.

(2) Tracking results as belonging to either the pre-burn-in period or
the post-burn-in period. Post-burn-in results will have special
handling (e.g. append-only within a category), with exact details TBD.

Test: builds
Bug: 1279686
Change-Id: I7c170ee3791256a1f3d7f0a20493e5ecf775cddf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3336923
Reviewed-by: Rachel Wong <wrong@chromium.org>
Commit-Queue: Amanda Deacon <amandadeacon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#951417}

chrome/browser/ui/BUILD.gn

@@ -1976,6 +1976,8 @@ static_library("ui") {
       "app_list/search/os_settings_provider.h",
       "app_list/search/ranking/answer_ranker.cc",
       "app_list/search/ranking/answer_ranker.h",
+      "app_list/search/ranking/burn_in_ranker.cc",
+      "app_list/search/ranking/burn_in_ranker.h",
       "app_list/search/ranking/category_item_ranker.cc",
       "app_list/search/ranking/category_item_ranker.h",
       "app_list/search/ranking/category_usage_ranker.cc",

chrome/browser/ui/app_list/search/ranking/burn_in_ranker.cc

@@ -0,0 +1,14 @@
+// Copyright 2021 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/ui/app_list/search/ranking/burn_in_ranker.h"
+
+namespace app_list {
+
+void BurnInRanker::UpdateResultRanks(ResultsMap& results,
+                                     ProviderType provider) {
+  // TODO(crbug.com/1279686): Implement.
+}
+
+}  // namespace app_list

chrome/browser/ui/app_list/search/ranking/burn_in_ranker.h

@@ -0,0 +1,39 @@
+// Copyright 2021 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_UI_APP_LIST_SEARCH_RANKING_BURN_IN_RANKER_H_
+#define CHROME_BROWSER_UI_APP_LIST_SEARCH_RANKING_BURN_IN_RANKER_H_
+
+#include "chrome/browser/ui/app_list/search/ranking/ranker.h"
+
+namespace app_list {
+
+// A ranker which implements a "burn-in" period for the updating of results.
+//
+// This ranker has two main tasks:
+//
+// 1) Delay the release of results until after the burn-in period has ended.
+// The purpose of this is to prevent the rapid and jittery updating of the
+// results list on initial display.
+//
+// 2) Mark each result as having arrived pre- or post-burn-in time.
+// Post-burn-in results may undergo special handling (e.g. append-only),
+// with exact details TBD.
+//
+// TODO(crbug.com/1279686): Update description when implemented.
+class BurnInRanker : public Ranker {
+ public:
+  BurnInRanker() = default;
+  ~BurnInRanker() override = default;
+
+  BurnInRanker(const BurnInRanker&) = delete;
+  BurnInRanker& operator=(const BurnInRanker&) = delete;
+
+  // Ranker:
+  void UpdateResultRanks(ResultsMap& results, ProviderType provider) override;
+};
+
+}  // namespace app_list
+
+#endif  // CHROME_BROWSER_UI_APP_LIST_SEARCH_RANKING_BURN_IN_RANKER_H_

chrome/browser/ui/app_list/search/ranking/ranker_delegate.cc

@@ -4,6 +4,7 @@
 
 #include "chrome/browser/ui/app_list/search/ranking/ranker_delegate.h"
 
+#include "chrome/browser/ui/app_list/search/ranking/burn_in_ranker.h"
 #include "chrome/browser/ui/app_list/search/ranking/filtering_ranker.h"
 #include "chrome/browser/ui/app_list/search/ranking/ftrl_category_ranker.h"
 #include "chrome/browser/ui/app_list/search/ranking/ftrl_result_ranker.h"
@@ -43,6 +44,9 @@ RankerDelegate::RankerDelegate(Profile* profile, SearchController* controller) {
   AddRanker(std::make_unique<RemovedResultsRanker>(
       PersistentProto<RemovedResultsProto>(
           state_dir.AppendASCII("removed_results.pb"), kNoWriteDelay)));
+
+  // Burn-in period for results.
+  AddRanker(std::make_unique<BurnInRanker>());
 }
 
 RankerDelegate::~RankerDelegate() {}