This extracts the lambda tasks to an own compilation unit and header
file. Additionally, it addresses the TODO to avoid templates and just
store the function to execute in an std::function.
Third, it provides the same functionality for pure (non-cancellable
non-idle) tasks.
Last, it removes the "Lambda" part from the methods, because we can
actually instantiate it with anything that is invocable (function
pointer, lambda, functor, ...).

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I2f613f5b15ee208f215bbf74bd6d1d41889fd637
Reviewed-on: https://chromium-review.googlesource.com/c/1328923
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57397}