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This issue manifested as all new account creations on Chrome OS crashing.
It was caused by the local device metadata being returned by DeviceSyncClient
not yet being initialized.

DeviceSyncClient now internally keeps track of whether it is "ready", that is,
whether its local device metadata and synced devices are available yet. Only
once it has loaded both sets of data will it mark itself as "ready", and inform
clients appropriately. This change also ensures that DeviceSyncClient's Observer
callbacks, OnEnrollmentFinished() and OnNewDevicesSynced(), are not called until
DeviceSyncClient is ready.

R=jhawkins@chromium.org, khorimoto@chromium.org

Bug: 856406
Change-Id: Idb6fdf123d4d90883f67bb1e88a617004926207e
Reviewed-on: https://chromium-review.googlesource.com/1121351Reviewed-by: James Hawkins <jhawkins@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572309}