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This change
* fixes some bugs in the way the PpdMetadataManager deals with
  cache expiration and
* adds unit tests to enforce the advertised conventions.

Bug: chromium:888189
Test: chromeos_unittests --gtest_filter='PpdMetadataManagerTest.*'
Change-Id: I6254fae91a158c93734930ba941d7b0404b8d115
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332037Reviewed-by: Luum Habtemariam <luum@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796056}