Expose the feature flag for the HistoryServiceUsesTaskScheduler experiment to be used in tests.

This is done to be able to prepare tests before eventually submitting a CL that flips the
experiment to ENABLED_BY_DEFAULT.

Bug: 661143
Change-Id: I6b614bb4400478a7e90de78562a54083af1b3bd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865596
Commit-Queue: Oliver Li <olivierli@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707378}

components/history/core/browser/history_service.cc

@@ -62,15 +62,17 @@
 using base::Time;
 
 namespace history {
-namespace {
 
-const base::Feature kHistoryServiceUsesTaskScheduler{
-    "HistoryServiceUsesTaskScheduler", base::FEATURE_DISABLED_BY_DEFAULT};
+namespace {
 
-static const char* kHistoryThreadName = "Chrome_HistoryThread";
+const char* kHistoryThreadName = "Chrome_HistoryThread";
 
 }  // namespace
 
+// static
+const base::Feature HistoryService::kHistoryServiceUsesTaskScheduler{
+    "HistoryServiceUsesTaskScheduler", base::FEATURE_DISABLED_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
 // the history service (otherwise we would have to manually AddRef and

components/history/core/browser/history_service.h

@@ -18,6 +18,7 @@
 #include "base/callback.h"
 #include "base/callback_list.h"
 #include "base/containers/flat_set.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ -87,6 +88,8 @@ class WebHistoryService;
 // as information about downloads.
 class HistoryService : public KeyedService {
  public:
+  static const base::Feature kHistoryServiceUsesTaskScheduler;
+
   // Must call Init after construction. The empty constructor provided only for
   // unit tests. When using the full constructor, |history_client| may only be
   // null during testing, while |visit_delegate| may be null if the embedder use