Load machine statistics with USER_BLOCKING priority on ChromeOS.

The NoBackgroundTasksTest.FirstNonEmptyPaintWithoutBackgroundTasks
test shows that loading machine statistics is on the critical path
of loading the NTP on ChromeOS. Therefore, it must have a
USER_BLOCKING priority to run as soon as possible.

Bug: 831835
Change-Id: Icdcf45b7380ad270441609b4b2fb6021c8ea5933
Reviewed-on: https://chromium-review.googlesource.com/1082608
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564524}

chromeos/system/statistics_provider.cc

@@ -477,9 +477,11 @@ void StatisticsProviderImpl::StartLoadingMachineStatistics(
   VLOG(1) << "Started loading statistics. Load OEM Manifest: "
           << load_oem_manifest;
 
+  // TaskPriority::USER_BLOCKING because this is on the critical path of
+  // rendering the NTP on startup. https://crbug.com/831835
   base::PostTaskWithTraits(
       FROM_HERE,
-      {base::MayBlock(), base::TaskPriority::BACKGROUND,
+      {base::MayBlock(), base::TaskPriority::USER_BLOCKING,
        base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
       base::BindOnce(&StatisticsProviderImpl::LoadMachineStatistics,
                      base::Unretained(this), load_oem_manifest));