[layered-api] Use private fields in key value store

Change-Id: I93838c7b02fb0fe64448a49b0bbbfde5903da2d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496278
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639132}
2 files changed