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There is no actual usage of this abstract class anymore.

Bug: 533069
Change-Id: I31ce5c99727a1ce11918b22a2252b3113c2c8878
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371683Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801161}