[runtime-call-stats] Allow null stats object in RuntimeCallStats::CorrectCurrentCounterId

This happens when RCS are enabled dynamically and the callsite is inside
the background parser.

BUG=chromium:760649

Change-Id: I216b955ed91d9c663ce3027aaa8ffb515bfe13ab
Reviewed-on: https://chromium-review.googlesource.com/740911Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49010}

src/counters.cc

@@ -514,8 +514,9 @@ void RuntimeCallStats::Add(RuntimeCallStats* other) {
 // static
 void RuntimeCallStats::CorrectCurrentCounterId(RuntimeCallStats* stats,
                                                CounterId counter_id) {
+  // When RCS are enabled dynamically there might be no stats or timer set up.
+  if (stats == nullptr) return;
   RuntimeCallTimer* timer = stats->current_timer_.Value();
-  // When RCS are enabled dynamically there might be no current timer set up.
   if (timer == nullptr) return;
   RuntimeCallCounter* counter = &(stats->*counter_id);
   timer->set_counter(counter);