[PM] Make PageAlmostIdleData use NodeAttachedData.

This cleans up PageNodeImpl substantially, and further hides the implementation of
PageAlmostIdleDecorator.

Change-Id: Ia9ae67aec1868f0ee7165a9bdbc07068b09ab714
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533123
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643353}
14 files changed