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This change implements the PpdMetadataManager class and provides unit
tests for the same.

Bug: chromium:888189
Change-Id: Ifb9672258a747722c3d48bd1a1f1095365555e49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261972
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784552}