Create RecentlyAudibleHelper.

A follow-up CL will refactor existing consumers of
WC::WasRecentlyAudible to use this API instead, before removing the
relevant content API. More details in the bug.

BUG=846374

Change-Id: I7a758b7fc4d01ca7e0a98676978d1de333829866
Reviewed-on: https://chromium-review.googlesource.com/1072748
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561848}
5 files changed