ResourceLoadScheduler: switch to use DCHECK instead of SECURITY_CHECK

Since I ensure that the assumption is correct, I will replace
one existing SECURITY_CHECK with DCHECK as I planned.

Bug: 768325
Change-Id: I1a653d8d3d3a7e62df537e9e4d3017798078a108
Reviewed-on: https://chromium-review.googlesource.com/681394Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504294}

third_party/WebKit/Source/platform/loader/fetch/ResourceLoadScheduler.cpp

@@ -78,9 +78,7 @@ void ResourceLoadScheduler::Shutdown() {
   if (!is_enabled_)
     return;
   auto* scheduler = context_->GetFrameScheduler();
-  // TODO(toyoshim): Replace SECURITY_CHECK below with DCHECK before the next
-  // branch-cut.
-  SECURITY_CHECK(scheduler);
+  DCHECK(scheduler);
   scheduler->RemoveThrottlingObserver(WebFrameScheduler::ObserverType::kLoader,
                                       this);
 }