Move PdfThumbnailer to chrome/services/pdf

The PdfThumbnailer code currently lives in chrome/services/printing, and
PrintingService provides access to it, but it is not used for printing.
As part of the work to create a PdfService, first move PdfThumbnailer to
its new location in chrome/services/pdf. Add BUILD.gn, DEPS, and OWNERS
files in the new directory.

Move the PdfThumbnailer from namespace printing to namespace pdf. Also
change build rules to ensure there is PDF support, not printing support.

Bug: 332743737
Change-Id: I874507a35fedb4052fdab2424466769201c0a1dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5426454


Reviewed-by: Alan Screen <awscreen@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283129}