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This CL adds an array of |Attachment| in MetadataMessageData so that the
back end can send the pp::VarArray of attachment information with the
rest of the metadata to the front end, and then stores it.

Bug: 177188
Change-Id: I676c8ecf86321eb0cbd2d95f328d76ed5eec0e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335032
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804344}