Actor: ExecutionEngine binds owner task in constructor

ExecutionEngine is owned by an ActorTask and must always have one set as
an owner. The protocol was that an ExecutionEngine is created first,
then an ActorTask, then the ExecutionEngine is passed into the ActorTask
and the task is set as the engine's owner.

Now that https://crrev.com/c/7524386 made it so that ExecutionEngine is
created by the ActorTask, we can simplify this exchange by simply
passing the ActorTask into ExecutionEngine's constructor.

There are tests which mock ExecutionEngine, and others which supply it
with a mock event dispatcher. This CL provides a mechanisms for tests to
inject an ExecutionEngine factory for testing. This allows tests to
override the creation of an ExecutionEngine without distorting the
initialization procedure.

Change-Id: Ie9859356659e6aaade7c142a5a7e26533aa8968e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7529055
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1577321}