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This change implements ParseUsbIndex().

Bug: chromium:888189
Test: chromeos_unittests --gtest_filter='PpdMetadataParserTest.*'
Change-Id: Ieec6f0f1a8d68d7ebf09b3d7ce971512896a5dfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335410
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797885}