Update comments after enabling HistoryServiceUsesTaskScheduler by default

This is the comments half of https://crrev.com/c/1960467 that was
reverted because of tests breakage. The actual flag flip is at
https://crrev.com/c/2000798. The tests have been fixed and this is conceptually
a reland.

Bug: 661143
Change-Id: Ic009bd84a8b4fe09f60967b90ff1434931e4734a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999228Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731654}

components/history/core/browser/history_service.cc

@@ -2,12 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// The history system runs on a background thread so that potentially slow
+// The history system runs on a background sequence so that potentially slow
 // database operations don't delay the browser. This backend processing is
 // represented by HistoryBackend. The HistoryService's job is to dispatch to
 // that thread.
 //
-// Main thread                       History thread
+// Main thread                       backend_task_runner_
 // -----------                       --------------
 // HistoryService <----------------> HistoryBackend
 //                                   -> HistoryDatabase

components/history/core/browser/history_service.h

@@ -468,7 +468,7 @@ class HistoryService : public KeyedService {
 
   // Generic Stuff -------------------------------------------------------------
 
-  // Schedules a HistoryDBTask for running on the history backend thread. See
+  // Schedules a HistoryDBTask for running on the history backend. See
   // HistoryDBTask for details on what this does. Takes ownership of |task|.
   virtual base::CancelableTaskTracker::TaskId ScheduleDBTask(
       const base::Location& from_here,
@@ -493,15 +493,15 @@ class HistoryService : public KeyedService {
 
   // Testing -------------------------------------------------------------------
 
-  // Runs |flushed| after bouncing off the history thread.
+  // Runs |flushed| after the backend has processed all other pre-existing
+  // tasks.
   void FlushForTest(base::OnceClosure flushed);
 
   // Designed for unit tests, this passes the given task on to the history
   // backend to be called once the history backend has terminated. This allows
-  // callers to know when the history thread is complete and the database files
-  // can be deleted and the next test run. Otherwise, the history thread may
-  // still be running, causing problems in subsequent tests.
-  //
+  // callers to know when the history backend has been safely deleted and the
+  // database files can be deleted and the next test run.
+
   // There can be only one closing task, so this will override any previously
   // set task. We will take ownership of the pointer and delete it when done.
   // The task will be run on the calling thread (this function is threadsafe).
@@ -511,9 +511,9 @@ class HistoryService : public KeyedService {
   // into the database. This assumes the URL doesn't exist in the database
   //
   // Calling this function many times may be slow because each call will
-  // dispatch to the history thread and will be a separate database
-  // transaction. If this functionality is needed for importing many URLs,
-  // callers should use AddPagesWithDetails() instead.
+  // post a separate database transaction in a task. If this functionality
+  // is needed for importing many URLs, callers should use AddPagesWithDetails()
+  // instead.
   //
   // Note that this routine (and AddPageWithDetails()) always adds a single
   // visit using the |last_visit| timestamp, and a PageTransition type of LINK,
@@ -594,9 +594,9 @@ class HistoryService : public KeyedService {
   // that is only set by unittests which causes the backend to not init its DB.
   bool Init(bool no_db, const HistoryDatabaseParams& history_database_params);
 
-  // Called by the HistoryURLProvider class to schedule an autocomplete, it
-  // will be called back on the internal history thread with the history
-  // database so it can query. See history_url_provider.h for a diagram.
+  // Called by the HistoryURLProvider class to schedule an autocomplete, it will
+  // be called back with the history database so it can query. See
+  // history_url_provider.h for a diagram.
   void ScheduleAutocomplete(
       base::OnceCallback<void(HistoryBackend*, URLDatabase*)> callback);
 
@@ -834,8 +834,8 @@ class HistoryService : public KeyedService {
   void NotifyProfileError(sql::InitStatus init_status,
                           const std::string& diagnostics);
 
-  // Call to schedule a given task for running on the history thread with the
-  // specified priority. The task will have ownership taken.
+  // Call to post a given task for running on the history backend sequence with
+  // the specified priority. The task will have ownership taken.
   void ScheduleTask(SchedulePriority priority, base::OnceClosure task);
 
   // Called when the favicons for the given page URLs (e.g.
@@ -858,17 +858,16 @@ class HistoryService : public KeyedService {
   // Cleanup() is called.
   scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
 
-  // This class has most of the implementation and runs on the 'thread_'.
-  // You MUST communicate with this class ONLY through the thread_'s
-  // task_runner().
+  // This class has most of the implementation. You MUST communicate with this
+  // class ONLY through |backend_task_runner_|.
   //
   // This pointer will be null once Cleanup() has been called, meaning no
-  // more calls should be made to the history thread.
+  // more tasks should be scheduled.
   scoped_refptr<HistoryBackend> history_backend_;
 
   // A cache of the user-typed URLs kept in memory that is used by the
   // autocomplete system. This will be null until the database has been created
-  // on the background thread.
+  // in the backend.
   // TODO(mrossetti): Consider changing ownership. See http://crbug.com/138321
   std::unique_ptr<InMemoryHistoryBackend> in_memory_backend_;