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This change brings the variants of the metadata type enum (internal to
the PpdMetadataManager class) in line with the style guide's guidance.

Bug: chromium:888189
Test: chromeos_unittests --gtest_filter='PpdMetadataManagerTest.*'
Change-Id: I9b87932b187c95f600db4caa3f945f9a9d6af5ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340634
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801972}