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Today both types are used on AttachmentService and other interfaces.
In this change I make all interfaces use AttachmentIdList.
AttachmentIdSet is only used in implementations and unittests.

R=maniscalco@chromium.org
BUG=
TEST=No observable behavior change.

Review URL: https://codereview.chromium.org/1035573002

Cr-Commit-Position: refs/heads/master@{#322130}