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Eliminate the middle level of the Operation <- OperationBase <-
{MakeCredential,GetAssertion}Operation class hierarchy. It was supposed
to allow the two concrete implementations share common behavior. But the
actual shared behavior was trivial, and so it just made the code more
annoying to read.

No functional changes intended.

Change-Id: I4de1107d17d702f5dfb1407127e09b1a2efeb75c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1965669Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#727548}