Update the timing of scheduler's OnTaskCompleted for microtasks

This updates OnTaskCompleted hook of Blink scheduler to flush V8 microtasks,
and updates its timing earlier, so that its elapsed time is counted as a
part of the main task.

Bug: 804661
Change-Id: I786d3b89687fb9bdad829e80c401a9258bfddab0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1517058
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657175}
29 files changed