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These changes are necessary to eventually integrate the zip archivers
unit tests.

Changes include:
- virtual -> override
- Un-inlining overridden functions
- Correcting types

BUG=889703

Change-Id: I83030c6b77dda026c4a3fee50df2ecbcedb898f6
Reviewed-on: https://chromium-review.googlesource.com/1250281Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Anand Mistry <amistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594972}