[v8::ItemParallelJob] Add an instant trace event to log num_tasks/num_items...

[v8::ItemParallelJob] Add an instant trace event to log num_tasks/num_items for ease of trace diagnosis.

R=mlippautz@chromium.org

Bug: chromium:651354
Change-Id: I5762813f6bd7b3c549f22ee8e43c59ca9edbe915
Reviewed-on: https://chromium-review.googlesource.com/904523
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51133}

src/heap/item-parallel-job.cc

@@ -63,6 +63,11 @@ void ItemParallelJob::Run(std::shared_ptr<Counters> async_counters) {
   const size_t num_items = items_.size();
   const size_t num_tasks = tasks_.size();
 
+  TRACE_EVENT_INSTANT2(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                       "ItemParallelJob::Run", TRACE_EVENT_SCOPE_THREAD,
+                       "num_tasks", static_cast<int>(num_tasks), "num_items",
+                       static_cast<int>(num_items));
+
   AsyncTimedHistogram gc_parallel_task_latency_histogram(
       async_counters->gc_parallel_task_latency(), async_counters);