[runtime-call-stats] Do not piggyback onto main thread's RCS in the background parser.

BUG=chromium:760649

Change-Id: If108830a1869238102e1c3475fe99e1ffb78d48d
Reviewed-on: https://chromium-review.googlesource.com/706140Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48654}

src/parsing/background-parsing-task.cc

@@ -37,6 +37,8 @@ BackgroundParsingTask::BackgroundParsingTask(
   info->InitFromIsolate(isolate);
   if (V8_UNLIKELY(FLAG_runtime_stats)) {
     info->set_runtime_call_stats(new (info->zone()) RuntimeCallStats());
+  } else {
+    info->set_runtime_call_stats(nullptr);
   }
   info->set_toplevel();
   std::unique_ptr<Utf16CharacterStream> stream(