Fix useless DCHECK in URLFetcherCore::StartURLRequest

The DCHECK on the pointer being non-null could be never hit,
as the pointer was dereferenced a couple of lines above.

BUG=none
TEST=existing tests

Review-Url: https://codereview.chromium.org/2732473003
Cr-Commit-Position: refs/heads/master@{#454471}

net/url_request/url_fetcher_core.cc

@@ -540,12 +540,12 @@ void URLFetcherCore::StartURLRequest() {
     return;
   }
 
+  DCHECK(request_context_getter_);
   if (!request_context_getter_->GetURLRequestContext()) {
     CancelRequestAndInformDelegate(ERR_CONTEXT_SHUT_DOWN);
     return;
   }
 
-  DCHECK(request_context_getter_.get());
   DCHECK(!request_.get());
 
   g_registry.Get().AddURLFetcherCore(this);