Worker: Check GetExecutionContext() first in DCHECK

Looks like GetExecutionContext() may return nulptr in terminate().

Bug: 884091
Change-Id: Ibd2756220a83ba490c0e23966650178e0b856423
Reviewed-on: https://chromium-review.googlesource.com/c/1264337
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597040}

third_party/blink/renderer/core/workers/dedicated_worker.cc

@@ -203,7 +203,7 @@ void DedicatedWorker::Start() {
 }
 
 void DedicatedWorker::terminate() {
-  DCHECK(GetExecutionContext()->IsContextThread());
+  DCHECK(!GetExecutionContext() || GetExecutionContext()->IsContextThread());
   context_proxy_->TerminateGlobalScope();
 }