[heap profiler] Refactor: make use of PostTaskWithTraitsAndReply

Get rid of an extra thread.

BUG=923459

Change-Id: I461ec8e437f0520274650a7922c29154fd57f23c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573171
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652175}
3 files changed