Guard DCHECK to avoid calling it after dispose().

AppendTask schedules this method, and it's possible
that after it's been scheduled, dispose() nulls out
the isolate_task_runner_. TakeNextTask will handle
this situation properly - if the runner is gone,
it returns Task(). So, moving the DCHECK should avoid the
following stack trace which I've seen twice thus far
when running tests with always DCHECK enabled.

Thank you for your suggestion for this fix, Dmitry.

[1:41:0725/182513.246104:FATAL:scoped_refptr.h(219)] Check failed: ptr_.

Change-Id: Ic24aef83876de05159a7cd9805ff8d2015d52629
Reviewed-on: https://chromium-review.googlesource.com/1151950Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578681}

third_party/blink/renderer/core/inspector/inspector_task_runner.cc

@@ -116,10 +116,11 @@ void InspectorTaskRunner::PerformSingleTask(Task task) {
 }
 
 void InspectorTaskRunner::PerformSingleTaskDontWait() {
-  DCHECK(isolate_task_runner_->BelongsToCurrentThread());
   Task task = TakeNextTask(kDontWaitForTask);
-  if (task)
+  if (task) {
+    DCHECK(isolate_task_runner_->BelongsToCurrentThread());
     PerformSingleTask(std::move(task));
+  }
 }
 
 void InspectorTaskRunner::V8InterruptCallback(v8::Isolate*, void* data) {