[Task Migration][Extensions] Update ApiResourceManager

api_resource_manager.h mentioned the FILE thread in a comment when
discussing resources on named threads (and theoretically would have
allowed its use). Update the comment to not mention the FILE thread and
add explicit notes that the only allowed threads are the IO and UI
threads, which are all we currently use, along with a note about how the
API could be expanded, if need be. To be safe, also add a static assert
to ensure that resources use no other threads.

Since we currently only need IO and UI threads, this CL doesn't
rearchitect anything unnecessarily.

Bug: 689520
Change-Id: I5d304fd9153865af7c9778d93b34f2e20ddb95a6
Reviewed-on: https://chromium-review.googlesource.com/591688
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491065}
1 file changed