Replace AttachmentStore's StoreAttachments with UploadAttachments.

Replace AttachmentStore's StoreAttachments with UploadAttachments.

This change is lays groundwork for making attachment upload operations
persistent.

Make GenericChangeProcessor responsible for writing attachments to the
AttachmentStore and calling UploadAttachments.  In a future CL, datatype
code will be responsible for writing attachments to the store.

Queue up attachments for upload inside of AttachmentService.  In a
future CL we'll add rate limiting, retry, and back-off logic.

Expose AttachmentService's AttachmentStore via GetStore method.

(This is a reland of issue 512413003.)

BUG=

Committed: https://crrev.com/38724a8f1286e8a3dd549ef187782aaede275d01
Cr-Commit-Position: refs/heads/master@{#293741}

[maniscalco, 2014-09-05 17:49:53]

maniscalco@chromium.org changed reviewers:
+ pavely@chromium.org

[maniscalco, 2014-09-05 17:49:54]

Pavel, would you please review this reland?  You have previously reviewed and
LGTM'd patch set #1.  Please review patch set #2.

Here's the original issue that was reverted,
https://codereview.chromium.org/512413003.  Turns out it raced with another
change that removed implicit conversion to * for scoped_refptr.

[pavely, 2014-09-05 17:54:09]

lgtm

[maniscalco, 2014-09-05 18:02:26]

The CQ bit was checked by maniscalco@chromium.org

[commit-bot: I haz the power, 2014-09-05 18:03:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/maniscalco@chromium.org/548533003/20001

[commit-bot: I haz the power, 2014-09-05 19:02:04]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-05 19:02:05]

Try jobs failed on following builders:
  android_clang_dbg_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_clang_d...)

[maniscalco, 2014-09-05 21:29:18]

The CQ bit was checked by maniscalco@chromium.org

[commit-bot: I haz the power, 2014-09-05 21:30:28]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/maniscalco@chromium.org/548533003/20001

[commit-bot: I haz the power, 2014-09-05 23:52:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-05 23:52:50]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)

[maniscalco, 2014-09-08 15:30:36]

The CQ bit was checked by maniscalco@chromium.org

[commit-bot: I haz the power, 2014-09-08 15:31:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/maniscalco@chromium.org/548533003/20001

[commit-bot: I haz the power, 2014-09-08 16:37:34]

Committed patchset #2 (id:20001) as d875ba580aaed07e027d9c74bd2f404430096af2

[commit-bot: I haz the power, 2014-09-10 03:46:47]

Patchset 2 (id:??) landed as
https://crrev.com/38724a8f1286e8a3dd549ef187782aaede275d01
Cr-Commit-Position: refs/heads/master@{#293741}