Revert of [Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

Revert of [Sync] Refactor AttachmentStore classes. Introduce concept of referrer. (patchset #2 id:20001 of https://codereview.chromium.org/986743004/)

Reason for revert:
I think this CL is responsible for tree closing compile failure https://build.chromium.org/p/chromium.mac/builders/iOS_Device/builds/26672/steps/compile/logs/stdio.

Original issue's description:
> [Sync] Refactor AttachmentStore classes. Introduce concept of referrer.
>
> In this change:
> - Move towards following class hierarchy: http://www.plantuml.com:80/plantuml/png/ZPBTJiCm38NlynIvv4TzWsaI525nCI5jkw-on1EZQHmvwQG9U7TC4-ZeKiITvNm-EiSExbv1Hxc6VOszYs24mDHQeG6xFNcGRqBAknYLVkd0nKr40l7nmqrU1deDeRUHYrfPkrEw3LmJx848YCkWqODfkECZBIOAZuHin9abWxUo9b0Hdjt38RGJyEhwZ4YZI9kJq_mmRp0pAPKnd7pGgG8t6usTHyVe7_FPtY3mbOth9ghGDjGxTnwlaEq-ySjv-KmCwZflxvVyE5bSIa4Mw7ZGyDHvAyHurPkgkhZgz9PuoNp75tDhAUZcJ68cwkATHyfXnFWn4_PFDsKuNLuKLrFodGS-0G00
> - Remove AttachmenService::GetStore. Now attachment store is owned by
>   model type, there is no need to get it from AttachmentService
> - Introduce AttachmentReferrer. There is no functionality behind it yet
>   and interface is not complete for it. These will come in the next change.
>
> BUG=457735
> R=maniscalco@chromium.org
> TEST=Not exposed in chrome. Only unit tests available (sync_unit_tests)
>
> Committed: https://crrev.com/6adcada8a799057c31c0f17550c9e2747a8df847
> Cr-Commit-Position: refs/heads/master@{#319733}

TBR=maniscalco@chromium.org,cjhopman@chromium.org,pavely@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=457735
Committed: https://crrev.com/cd8db1dfaf165cbca88f38ecf50a5cacc09959b1
Cr-Commit-Position: refs/heads/master@{#319760}

[Bence, 2015-03-09 23:08:49]

Created Revert of [Sync] Refactor AttachmentStore classes. Introduce concept of
referrer.

[commit-bot: I haz the power, 2015-03-09 23:10:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/996473005/1

[commit-bot: I haz the power, 2015-03-09 23:13:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-09 23:14:32]

Patchset 1 (id:??) landed as
https://crrev.com/cd8db1dfaf165cbca88f38ecf50a5cacc09959b1
Cr-Commit-Position: refs/heads/master@{#319760}