Check for nullptr in PerformNextQueuedGlobalMemoryDump.

The function is both queued as a task, and called synchronously. It makes the
assumption that there exists a next request, but there are race conditions that
could cause there to be no next request.

Bug: 821177
Change-Id: Id0abf1b07bca812ad976184de87cc22d42fb18b9
Reviewed-on: https://chromium-review.googlesource.com/966481
Commit-Queue: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544292}

services/resource_coordinator/memory_instrumentation/coordinator_impl.cc

@@ -341,6 +341,9 @@ void CoordinatorImpl::PerformNextQueuedGlobalMemoryDump() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   QueuedRequest* request = GetCurrentRequest();
 
+  if (request == nullptr)
+    return;
+
   std::vector<QueuedRequestDispatcher::ClientInfo> clients;
   for (const auto& kv : clients_) {
     auto client_identity = kv.second->identity;