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- AttachmentDownloader interface is similar to AttachmentUploader interface.
  No implementation yet.
- AttachmentStore::Read guarantee should be stronger. It should attempt
  to read all attachments and return list of attachment ids that can't
  be loaded locally. Those need to be downloaded from server.

R=maniscalco@chromium.org
BUG=376073

Review URL: https://codereview.chromium.org/293143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272585 0039d316-1c4b-4281-b951-d872f2087c98