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The LevelDB folder backing ModelTypeStore is inside Sync Data folder.
While this change is being driven by race conditions from sync trying
to delete this folder while it is being used, this setup is fundamental
flawed. The ModelTypeStores will hold model type specific information
that needs to be persistent across sign outs and disabling sync.

The approach this CL takes to fix this problem is the modify the
deletion logic to only affect the Directory (sqllite3) files. This
means that the Sync Data folder will still exist even when sync is off.

BUG=673508,673887

Review-Url: https://codereview.chromium.org/2568743004
Cr-Commit-Position: refs/heads/master@{#439835}