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- PpdResolution holds the state of active PpdReference resolutions.
- Will be incorporated in a later class, PpdReferenceTracker.
- Adds unit test for this class.

Bug: 932202
Test: --gtest_filter=PpdResolutionStateTest.*
Change-Id: I9135e24fd883ae06cb483dc7c12ae7d996564bfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565377
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650542}