Sync: Handle empty (implicit) ParentId in GetPredecessorId and GetSuccessorId

Sync: Handle empty (implicit) ParentId in GetPredecessorId and GetSuccessorId

GetSuccessorId and GetPredecessorId access all children of the
current node's parent in order to get to the next/previous node.
With implicit parent ID these functions can't get to the parent.
The reasons we didn't run into this problem before are
a) The server feature that will stop setting parent ID isn't
   enabled yet so there wasn't a chance to expose this yet.
b) There wasn't enough test coverage.

The fix is to use the ParentChildIndex to resolve the implicit
(unset) parent ID. Added a new method called GetSiblings at
the Directory level that does just that. Also added some
unit test coverage.

BUG=438313
Committed: https://crrev.com/12cf7b569a3dc4cbf0f5ee88377bae8e47fc6653
Cr-Commit-Position: refs/heads/master@{#320922}

[stanisc, 2015-03-16 18:47:58]

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

[stanisc, 2015-03-16 18:47:59]

PTAL

[pavely, 2015-03-16 20:57:43]

lgtm

[stanisc, 2015-03-17 04:48:34]

On 2015/03/16 20:57:43, pavely wrote:
> lgtm

FYI, added one extra unit test in directory_unittest.cc.

[stanisc, 2015-03-17 15:38:18]

The CQ bit was checked by stanisc@chromium.org

[stanisc, 2015-03-17 15:38:18]

The patchset sent to the CQ was uploaded after l-g-t-m from pavely@chromium.org
Link to the patchset: https://codereview.chromium.org/1008993004/#ps20001
(title: "Added extra unit test.")

[commit-bot: I haz the power, 2015-03-17 15:38:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1008993004/20001

[commit-bot: I haz the power, 2015-03-17 15:42:11]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-17 15:42:58]

Patchset 2 (id:??) landed as
https://crrev.com/12cf7b569a3dc4cbf0f5ee88377bae8e47fc6653
Cr-Commit-Position: refs/heads/master@{#320922}