Sync: support implicit permanent folders in commits.

Sync: support implicit permanent folders in commits.

Traversal::AddUncommittedParentsAndTheirPredecessors walks the
hierarchy to sort commits which is not necessary for any types
except bookmarks.
We've seen some crashes here when implicit permanent folders
were turned on alpha server. I reproduced the crash by planting
an empty parent ID in the database.

The fix avoids traversing the hierarchy for committed items with
unset parent IDs. Verified that this has stopped the crash
locally and successfully committed the item.

BUG=438313
Committed: https://crrev.com/8d56b3a917a92081971e361b1939deff131149fd
Cr-Commit-Position: refs/heads/master@{#321472}

[stanisc, 2015-03-19 22:08:10]

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

[pavely, 2015-03-19 23:40:52]

lgtm

[stanisc, 2015-03-19 23:45:21]

The CQ bit was checked by stanisc@chromium.org

[commit-bot: I haz the power, 2015-03-19 23:45:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1023843002/1

[commit-bot: I haz the power, 2015-03-19 23:59:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 00:00:25]

Patchset 1 (id:??) landed as
https://crrev.com/8d56b3a917a92081971e361b1939deff131149fd
Cr-Commit-Position: refs/heads/master@{#321472}