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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

Review URL: https://codereview.chromium.org/1023843002

Cr-Commit-Position: refs/heads/master@{#321472}