[TaskScheduler]: Implement ExecutionEnvironment.

In order to use TaskSource from TaskTracker instead of Sequence,
the sequence specific data must be encoded in a new struct
ExecutionEnvironment and returned from a virtual method.

Bug: 839091
Change-Id: I64d06c540a1eb9a7f25036ee73e7a34f024f1dbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524523
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641989}
5 files changed