Redirect HistoryService thread to TaskScheduler via a field trial.

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index 290ade81d15171854b5199e45944d0bc1e4d5f1f..75c03736ef1729a5767f21a443ed980a25d8a392 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -122,6 +122,7 @@
 #include "components/device_event_log/device_event_log.h"
 #include "components/flags_ui/pref_service_flags_storage.h"
 #include "components/google/core/browser/google_util.h"
+#include "components/history/core/browser/history_service.h"
 #include "components/language_usage_metrics/language_usage_metrics.h"
 #include "components/metrics/call_stack_profile_metrics_provider.h"
 #include "components/metrics/metrics_reporting_default_state.h"
@@ -330,13 +331,21 @@ void MaybeInitializeTaskScheduler() {
   if (!task_scheduler_util::InitializeDefaultTaskScheduler(variation_params))
     return;
 
-  // TODO(gab): Remove this when http://crbug.com/622400 concludes.
+  // TODO(gab): Remove this when https://crbug.com/622400 concludes.
   const auto sequenced_worker_pool_param =
       variation_params.find("RedirectSequencedWorkerPools");
   if (sequenced_worker_pool_param != variation_params.end() &&
       sequenced_worker_pool_param->second == "true") {
     base::SequencedWorkerPool::RedirectToTaskSchedulerForProcess();
   }
+
+  // TODO(fdoray): Remove this when https://crbug.com/661143 concludes.

[robliao, 2016/11/08 22:21:50]

Add this to ios_chrome_main_parts.mm.

Side thought, can be done in a separate CL: Might be a good idea to centralize
the redirection portion in components/task_scheduler_utils.

+  const auto history_service_param =
+      variation_params.find("RedirectHistoryService");
+  if (history_service_param != variation_params.end() &&
+      history_service_param->second == "true") {
+    history::HistoryService::RedirectToTaskSchedulerForProcess();
+  }
 }
 
 // Returns the new local state object, guaranteed non-NULL.