[runtime-call-stats] Fix NPE in RuntimeCallStats::Dump

The runtime_call_stats_ can be NULL on a worker thread when RCS
are enabled dynamically.

BUG=v8:7043

Change-Id: I1d26ae76c46955e017c82037b4a015ad86ba2f77
Reviewed-on: https://chromium-review.googlesource.com/755419Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49245}

src/parsing/parser.cc

@@ -3505,8 +3505,9 @@ void Parser::ParseOnBackground(ParseInfo* info) {
     if (result != nullptr) *info->cached_data() = logger.GetScriptData();
     log_ = nullptr;
   }
-  if (FLAG_runtime_stats &
-      v8::tracing::TracingCategoryObserver::ENABLED_BY_TRACING) {
+  if (runtime_call_stats_ &&
+      (FLAG_runtime_stats &
+       v8::tracing::TracingCategoryObserver::ENABLED_BY_TRACING)) {
     auto value = v8::tracing::TracedValue::Create();
     runtime_call_stats_->Dump(value.get());
     TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats"),