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SyncExtensionFunctions, AsyncExtensionFunctions,
ChromeSyncExtensionFunctions, and ChromeAsyncExtensionFunctions are
deprecated.

Remove SyncExtensionFunctions from:
- management
- metrics_private
- system_private
- networking_private
- power

BUG=634140

Review-Url: https://codereview.chromium.org/2261313002
Cr-Commit-Position: refs/heads/master@{#414284}