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This change implements PpdProvider::ReverseLookup(). This change also
corrects a logic error in PpdMetadataManager, in which I overzealously
assumed that callers always supply entries that we _must_ find in the
corresponding metadata (untrue: caller can simply supply gibberish, and
we should fail gracefully).

Bug: chromium:888189
Test: chromeos_unittests --gtest_filter='PpdProviderTest.ReverseLookup'
Test: chromeos_unittests --gtest_filter='PpdMetadataManagerTest.*'
Change-Id: Iebf70a0ad8d60044c031178a27bd3f5c83f32b06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357249
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805895}