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Add class comments and some renames for PaintCache. Following up from:
https://chromium-review.googlesource.com/c/chromium/src/+/1321190/4

R=enne@chromium.org

Bug: 894200
Change-Id: Ie8a0629071f8156e26945be7e9c61e04742158e5
Reviewed-on: https://chromium-review.googlesource.com/c/1336040Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608298}