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GetOrDownloadAttachments creates state object that tracks set of attachments
for which requests are still pending. Once AttachmentStore::Read finishes 
AttachmentServiceImpl calls AttachmentDownloader to download attachments that 
were not found locally. 

Once all calls to Downloader finish consumer callback is called with results.

I used AttachmentIdSet in this code but didn't update AttachmentIdList to 
AttachmentIdSet in other places. I'd rather do it in separate change.

R=maniscalco@chromium.org
BUG=376073

Review URL: https://codereview.chromium.org/307783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274164 0039d316-1c4b-4281-b951-d872f2087c98