Hook up chrome.printerProvider.onPrintRequested

Hook up chrome.printerProvider.onPrintRequested

This adds PrinterProviderAPI service which implements logic for dispatching
onPrintRequested event, and keeps track of pending event callbacks.

Adds PrinterProviderInternalAPI service and
PrinterProviderInternalAPIObserver. The internal API service keeps observer list,
which gets notified when printerProviderInternal API functions get called.

PrinterProviderAPI observes PrinterProviderInternalAPI service for when
print request result is reported, and runs the appropriate callback.
The callback is identified by the request_id param passed to the
internal API function (whose value is the one passed to chrome.printerProvider
event when it's dispatched).

BUG=408772

Review URL: https://codereview.chromium.org/853263002

Cr-Commit-Position: refs/heads/master@{#312506}