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This change prepares the client to deal with implicit permanent folders
in Sync server updates. That includes the following:
- Expect type root folders to not come from the server on the initial
  sync. The client auto-creates type root folders for all types except
  Bookmarks and Nigori when progress marker changes from empty to
  non-empty. These folders are created as local nodes (with client IDs)
  that aren't expected to sync back to the server.
- Expect empty parent IDs in updates for both new and existing items.
- Because the client code updates first, the client must expect server
  to override locally created type root folders. To enable that the
  directory update code that matches update entities to local entities
  was updated to look at server unique tags in addition to client
  unique tags. Later when the server stops generating folders we
  should be able to remove that code.
- Added some extra special cases in Directory::CheckTreeInvariants to
  deal with client side created type root folders.
- Added / modified a few tests to cover cases with implicit parent IDs
  in updates.

BUG=438313

Review URL: https://codereview.chromium.org/867793003

Cr-Commit-Position: refs/heads/master@{#313977}