[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}