Extract code handling PrinterProviderAPI from PrintPreviewHandler
Introduces an interface that abstracts handling requests that should be
forwarded to printer provider API from PrintPreviewHandler.
The interface implementation currently doesn't do much, just invokes
the PrinterProviderAPI, and it makes canceling callbacks to pending
requests easier. This will change when print requests are implemented.
BUG=408772
Review URL: https://codereview.chromium.org/
919873002
Cr-Commit-Position: refs/heads/master@{#316017}