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This CL adds event handlers for downloading attachments, and replace
|attachmentToken| with |messageId| in |SaveAttachmentDataMessageData|
since now the controller uses |messageId| to locate the resolver.

Bug: 177188
Change-Id: I9f88c916d5eff997a2b03c91bf646c8dfa6a66e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337408Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807665}