[Extensions Offscreen Documents] Add core lifetime enforcement

Add the basic infrastructure for lifetime enforcement of offscreen
documents. Since offscreen documents can be created for different
reasons, this involves the introduction of a base "lifetime enforcer"
class, which can then be subclassed to monitor for different types of
behavior.

These lifetime enforcers will be generated by a new factory (which will
create the lifetime enforcers corresponding to the document's reasons),
and share the lifetime with the document. The lifetime enforcers are
managed by the OffscreenDocumentManager; when all lifetime enforcers
report that a document is no longer active, the manager terminates the
document.

This CL adds the basic implementation of the lifetime enforcers and the
factory (along with testing utilities) and tests for the basic lifetime
enforcer behavior (i.e., that the document is properly terminated when
the enforcers indicate it is inactive).

Bug: 1339382
Change-Id: Ia51ccd33147778777d8993945fdb45503de522b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4000752
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1068283}