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This change implements
PpdMetadataManager::FindAllEmmsAvailableInIndex(), which seeks out PPD
PPD information via the forward index metadata for a given list of
effective-make-and-model strings. This is the first method in a series
needed to implement PpdProvider::ResolvePpdReference().

Bug: chromium:888189
Test: chromeos_unittests --gtest_filter='PpdMetadataManagerTest.*'
Change-Id: I465d7fa6589aa519ed80709a2320d8c1ec4b0e74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333582Reviewed-by: Luum Habtemariam <luum@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797876}