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Previously printing code was in a mix of chromeos:: and
chromeos::printing.  This standardizes everything on chromeos::,
removing the chromeos::printing namespace completely.

These changes are almost entirely mechanical.  I did rename the factory function "CreateProvider" to the less generic "CreatePpdProvider" since it's now in ::chromeos directly.

BUG=702710

Review-Url: https://codereview.chromium.org/2975013002
Cr-Commit-Position: refs/heads/master@{#487334}