[PM] Create bare-bones public interface.

Further CLs will expand the public interfaces one by one.

BUG=958115

Change-Id: Iedf6382e40bfc02af3d9404a5b380f97d9d02a3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594928
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Auto-Submit: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656812}