cros: Fix DCHECK for rlz_lib::UpdateExistingAccessPointRlz

|UpdateExistingAccessPointRlz| involves file I/O, so it should be
posted on a task runner that has MayBlock() in its TaskTraits.
|background_task_runner_| serves this purpose.

Bug: 906293
Change-Id: If36215767a0dbcfbad56c0432b16b584f6185376
Reviewed-on: https://chromium-review.googlesource.com/c/1340607Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609428}

components/rlz/rlz_tracker.cc

@@ -303,7 +303,12 @@ bool RLZTracker::Init(bool first_run,
       delegate_->IsBrandOrganic(reactivation_brand_)) {
     SYSLOG(INFO) << "RLZ is disabled";
   } else {
-    rlz_lib::UpdateExistingAccessPointRlz(brand_);
+    background_task_runner_->PostTask(
+        FROM_HERE, base::BindOnce(
+                       [](const std::string& brand) {
+                         rlz_lib::UpdateExistingAccessPointRlz(brand);
+                       },
+                       brand_));
   }
 #endif