Add `kPrintedPdf` holding space item type.

The new holding space item type will be used when adding holding space
items to the model when the user prints a file to PDF.

Note that the new service API is not wired up in this CL, that logic
will be added in a follow up. This CL does not change any user-facing
behavior.

As a new holding space item type has been added, holding space
histograms are updated accordingly.

Bug: 1184422, 1196086
Change-Id: I0b5faedd1414b9ffc8b4a53959e9a80f60c53542
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2864509
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#878580}