Use USER_BLOCKING priority in SQLitePersistentCookieStore.

We have verified that tasks posted by SQLitePersistentCookieStore are
on the critical path of a page load. According to the documentation in
task_traits.h, being on the critical path of a page load requires
USER_BLOCKING priority.

We keep this as an experiment because we can to assess the impact of
this change when the WindowsThreadModeBackground feature is enabled.

Bug: 872820, 878222
Change-Id: Iec368de8f59b34703688d8f8811aedbeddc50a8d
Reviewed-on: https://chromium-review.googlesource.com/c/1303039Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603308}

net/extras/sqlite/sqlite_persistent_cookie_store.cc

@@ -44,9 +44,14 @@ using base::Time;
 namespace {
 
 // Changes the recommended priority of |background_task_runner| to
-// USER_BLOCKING.
-const base::Feature kCookieStorePriorityBoost{
-    "CookieStorePriorityBoost", base::FEATURE_DISABLED_BY_DEFAULT};
+// USER_BLOCKING. ENABLED_BY_DEFAULT because we have verified that this is on
+// the critical path of page load. Still an experiment to allow assessing the
+// impact when the WindowsThreadModeBackground feature is enabled.
+//
+// TODO(fdoray): Remove this feature when experiment is complete.
+// https://crbug.com/872820
+const base::Feature kCookieStorePriorityBoost{"CookieStorePriorityBoost",
+                                              base::FEATURE_ENABLED_BY_DEFAULT};
 
 std::unique_ptr<base::Value> CookieKeyedLoadNetLogCallback(
     const std::string& key,