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As we turn the feature EnableSyncUSSDeviceInfo to handle DeviceInfo
with a USS approach instead of via Directory, we want to have the bots
testing the new code paths.

There were several issues that were fixed before this patch. Here we're
only making two changes to fix failing tests.

1. Python sync server implementation is relaxing its restrictions about
progress marker. It seemed arbitrarily and pedantic for it to treat
empty tokens and non-present tokens differently, so now starts both as
first time syncs. This more accurately reflects how the real server
handles this case.

2. EnableDisableSingleClientTest no longer tests USS types. As we add
more USS  types we'll unfortunately have to update this code to exclude
those types as well. There isn't a good USS equivalent to checking for
top level nodes to see if a type has actual sync data. There is no
directory and there are no top level nodes. When USS is completely
rolled out we may just want to remove this test class.

BUG=650725

Review-Url: https://codereview.chromium.org/2568543004
Cr-Commit-Position: refs/heads/master@{#439870}