Sync: ParentChildIndex fix for out of order deletion of entries by PurgeEntriesWithTypeIn.

Sync: ParentChildIndex fix for out of order deletion of entries by PurgeEntriesWithTypeIn.

This is an incremental fix for 438313 that addresses the two issues mentioned here:
https://code.google.com/p/chromium/issues/detail?id=438313#c24
This issues are reproducible only with the alpha server with
implicit permanent folders enabled. I added a test case for
one of them - the out of order deletion which results in
deleting the permanent folder for a datatype before some of
the items of the same datatype. That occurs only in one
particular corner case when a failed datatype need to be
purged and re-downloaded.

BUG=438313
Committed: https://crrev.com/0ddf23080ef7d1317485e76d77a69e857b697580
Cr-Commit-Position: refs/heads/master@{#330657}

[stanisc, 2015-05-19 01:16:47]

Patchset #1 (id:1) has been deleted

[stanisc, 2015-05-19 18:37:57]

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

[stanisc, 2015-05-19 18:42:47]

PTAL

[pavely, 2015-05-19 21:28:25]

lgtm

[stanisc, 2015-05-19 23:59:11]

The CQ bit was checked by stanisc@chromium.org

[commit-bot: I haz the power, 2015-05-20 00:21:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139883007/20001

[commit-bot: I haz the power, 2015-05-20 00:29:11]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-05-20 00:30:06]

Patchset 1 (id:??) landed as
https://crrev.com/0ddf23080ef7d1317485e76d77a69e857b697580
Cr-Commit-Position: refs/heads/master@{#330657}