[Sync] Introduce AttachmentStoreForSync class

[Sync] Introduce AttachmentStoreForSync class

In this change:
- Add AttachmentStoreForSync class. It has functions that will be called by AttachmentServiceImpl to manage sync’s references to attachments.
- Change AttachmentServiceImpl to call those functions. The rule is that for all attachments that we start uploading call AddSyncReference, for all attachments that we no longer upload (for whatever reason) call DropSyncReference.

Couple of notes:
- There is no storage implementation for those functions, it will come in the next change.
- There is growing confusion between AttachmentIdSet and AttachmentIdList. I’ll resolve this next week in a separate change.

BUG=457735
R=maniscalco@chromium.org
TEST=No user facing behavior. Change covered by sync_unit_tests.

Committed: https://crrev.com/dc577c86a31224a121b37e0f46e6de8f5e7f6435
Cr-Commit-Position: refs/heads/master@{#322263}

[pavely, 2015-03-24 23:22:45]

Nick, 
I made rename changes that we discussed the other day.
PTAL.

[maniscalco, 2015-03-25 23:37:39]

lgtm

[pavely, 2015-03-25 23:41:18]

The CQ bit was checked by pavely@chromium.org

[commit-bot: I haz the power, 2015-03-25 23:42:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1002263005/40001

[commit-bot: I haz the power, 2015-03-25 23:48:05]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-03-25 23:48:59]

Patchset 3 (id:??) landed as
https://crrev.com/dc577c86a31224a121b37e0f46e6de8f5e7f6435
Cr-Commit-Position: refs/heads/master@{#322263}

[JChim51, 2015-04-25 03:37:54]

jchim51@gmail.com changed reviewers:
+ JChim51@gmail.com