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This CL renames:
- Verifier to CurrentPageVerifier.
- VerifierDelegate to Verifier.
- TwaVerifierDelegate to TwaVerifier.
- corresponding test classes.

Bug: 1017114
Change-Id: I8c9e9688ba1eca415bc3c4b4951e2839db458c89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899987Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713158}