[CUPS Printing] Add feature flag for PPD attribute support.

This might take more than milestone to finish so putting this behind a
feature flag.

Bug: 964919
Change-Id: I11acaa377b282d9acd9788b2e1a54bf0d2297511
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617762
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662221}
8 files changed