Add back a threading DCHECK to ThrottlingController.

Bug: 960874
Change-Id: Iecc7c1a4e22dcc79ac7de28cfc7090339b883f38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151889
Commit-Queue: Eric Roman <eroman@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Auto-Submit: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759501}

services/network/throttling/throttling_controller.cc

@@ -57,6 +57,9 @@ bool ThrottlingController::HasInterceptor(
   // Null |instance_| means there is no network condition registered.
   if (!instance_)
     return false;
+
+  DCHECK_CALLED_ON_VALID_THREAD(instance_->thread_checker_);
+
   return instance_->interceptors_.find(throttling_profile_id) !=
          instance_->interceptors_.end();
 }