Redirect HistoryService thread to TaskScheduler via a field trial.

components/history/core/browser/history_service.h

Index: components/history/core/browser/history_service.h
diff --git a/components/history/core/browser/history_service.h b/components/history/core/browser/history_service.h
index 430037133854ba0d0e65a1ab87763162f4f13eaa..d7c9bc49556bf47677b464af5b3ea7c1eeaf3f49 100644
--- a/components/history/core/browser/history_service.h
+++ b/components/history/core/browser/history_service.h
@@ -22,6 +22,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
+#include "base/sequenced_task_runner.h"
 #include "base/strings/string16.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "base/threading/thread_checker.h"
@@ -93,6 +94,14 @@ class HistoryService : public syncer::SyncableService, public KeyedService {
                  std::unique_ptr<VisitDelegate> visit_delegate);
   ~HistoryService() override;
 
+  // When this is called, all HistoryService instances in this process will run
+  // their HistoryBackend tasks on a TaskScheduler sequence rather than on a
+  // dedicated thread. This must be called before instantiating the first
+  // HistoryService in the process.
+  // TODO(fdoray): Remove once the HistoryService redirection experiment
+  // concludes. https://crbug.com/661143
+  static void RedirectToTaskSchedulerForProcess();
+
   // Initializes the history service, returning true on success. On false, do
   // not call any other functions. The given directory will be used for storing
   // the history files.
@@ -795,9 +804,15 @@ class HistoryService : public syncer::SyncableService, public KeyedService {
 
   base::ThreadChecker thread_checker_;
 
-  // The thread used by the history service to run complicated operations.
-  // |thread_| is null once Cleanup() is called.
-  base::Thread* thread_;
+  // The thread used by the history service to run HistoryBackend operations.
+  // Intentionally not a BrowserThread because the sync integration unit tests
+  // need to create multiple HistoryServices which each have their own thread.
+  // Nullptr if TaskScheduler is used for backend operations.
+  std::unique_ptr<base::Thread> thread_;
+
+  // The TaskRunner to which HistoryBackend tasks are posted. Nullptr once
+  // 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