Enable HistoryServiceUsesTaskScheduler by default

This is the code half of https://crrev.com/c/1960467 that was
reverted because of tests breakage. Comments will be uploaded
in a separate CL. The tests have been fixed and this is conceptually
reland.

Bug: 661143
Change-Id: Ic61c6a77cf232330c575f1ac5621c1393b5db1d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000798Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731656}

components/history/core/browser/history_service.cc

@@ -71,7 +71,7 @@ const char* kHistoryThreadName = "Chrome_HistoryThread";
 
 // static
 const base::Feature HistoryService::kHistoryServiceUsesTaskScheduler{
-    "HistoryServiceUsesTaskScheduler", base::FEATURE_DISABLED_BY_DEFAULT};
+    "HistoryServiceUsesTaskScheduler", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Sends messages from the backend to us on the main thread. This must be a
 // separate class from the history service so that it can hold a reference to