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Embedders can register a TaskExecutor to handle execution of Tasks
posted via the post_task.h API with specific extension TaskTraits.

The TaskScheduler becomes the default TaskExecutor.

Related patch introducing TaskTraits extensions:
https://chromium-review.googlesource.com/c/chromium/src/+/1169179
Design doc:
https://docs.google.com/document/d/1SGy9VTXUwyXEX_yBZ0ukFAnS8B0hDeMUJD-1iALaE-Q/edit?usp=sharing

TBR=pasko@chromium.org,primiano@chromium.org,imcheng@chromium.org,dominickn@chromium.org

Bug: 867421
Change-Id: I4083848431cca17aab3b6a411170ea32e5503776
Reviewed-on: https://chromium-review.googlesource.com/1175829Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584709}