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This changes two main things.  First, and most significantly, the
way we resolve a PpdReference is unified -- instead of trying
a make-and-model string or trying usb ids, now we have a single
function to which you supply everything you know about the printer
and it gives you back a PpdReference or a failure.

Second, the way we support UI make, model listings changes.  When
the user asks for a list of supported printers from a manufacturer,
instead of getting back just the names, now the api returns the
names paired with the corresponding PpdReference.  Before, the
user had to call back into the ppd provider to get the reference, which
meant we had a weird "have to call this before that" sort of constraint
which this removes.

All callsites and tests updated as well.

Bug: 744996
Change-Id: Ied385c087e4c3d6c1b21b924fcea772c786074c5
Reviewed-on: https://chromium-review.googlesource.com/580007
Commit-Queue: Justin Carlson <justincarlson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488716}