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This CL adds the necessary wiring to make devtools aware of postTask to
show async callstacks, similar to setTimeout and requestIdleCallback.

Bug: 979017
Change-Id: I960b13d1c13f171dbc033b8e79bff3768e65c347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025870Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736504}